Fuel cell system and operation method thereof

ABSTRACT

A fuel cell system ( 100   a ) includes a fuel cell ( 11 ), a fuel gas supplier ( 16 ) configured to supply a fuel gas; an oxidizing gas supplier ( 17 ) configured to supply an oxidizing gas, and a controller ( 20 ), and further includes a moisture supply mechanism ( 27, 28, 29 ) configured to supply moisture to at least one of an anode and a cathode of the fuel cell, wherein the controller is configured to control the moisture supply mechanism based on at least one of either a dew point of the fuel gas or information related to the dew point and either a dew point of the oxidizing gas or information related to the dew point as to increase at least either the dew point of the fuel gas or the dew point of the oxidizing gas, in order to supply moisture to at least one of the anode or the cathode of the fuel cell before cutting off electric connection between the fuel cell and a load, and is configured to thereafter cut off the electric connection between the fuel cell and the load.

TECHNICAL FIELD

The present invention relates to a fuel cell system including polymerelectrolyte fuel cells configured to generate electric power using fuelgas containing hydrogen and oxidizing gas containing oxygen. Thisinvention also relates to an operation method for such a fuel cellsystem.

In recent years, various studies have been conducted in an effort tofind a way to accomplish the practical use of a fuel cell system (e.g.,the practical use of the fuel cell system as an electric powergenerating system intended for a mobile object such as an electricvehicle, and a household cogeneration system) which at least includes apolymer electrolyte fuel cell, a fuel gas supplier for supplying fuelgas containing moisture (water and/or water vapor) to an anode of thepolymer electrolyte fuel cell and an oxidizing gas supplier forsupplying oxidizing gas containing moisture to a cathode of the polymerelectrolyte fuel cell.

Of these studies conducted, the development of technologies for ensuringsufficient energy conversion efficiency of the fuel cell system is oneof the important subjects to be studied. Therefore, as a fuel cellsystem and an operation method thereof able to secure sufficient energyconversion efficiency, there is proposed a fuel cell system operationmethod or a fuel cell system operated based thereon, wherein the fuelcell system is operated to generate electric power in a certainoperational condition that meets the interrelations: Tcell>Tda andTcell>Tdc (hereinafter, this operational condition is referred to as a“low humidification state” if necessary) where Tda is the fuel gas dewpoint, Tdc is the oxidizing gas dew point, and Tcell is the polymerelectrolyte fuel cell's temperature.

On the other hand, in a method of operating a fuel cell system, there isno need to put the fuel cell system in operation when neither electricenergy nor heat energy is required. Therefore, in a typical fuel cellsystem operation method, there is employed a start-up/shutdownoperational mode in which the fuel cell system is started up wheneverelectric energy and heat energy are needed while on the other hand it isshut down whenever electric energy and heat energy are no longer needed.

Incidentally, in a fuel cell system that employs such astart-up/shutdown operational mode, the polymer electrolyte fuel cellchanges its state from closed circuit state to open circuit state if thepolymer electrolyte fuel cell in operation to generate electric power isshut down. Here, if, in the case where the fuel cell system is operatedin a low humidification state, the polymer electrolyte fuel cell changesits state from closed circuit state to open circuit state in associationwith the shutdown of the electric power generation operation of the fuelcell system, the degradation of the polymer electrolyte membrane becomesconsiderably worse due to a change in the wet state of the polymerelectrolyte membrane caused by the fact that water is no longer formed.As a result, in such a fuel cell system, fluoride ions, i.e., decomposedsubstances of the polymer electrolyte membrane, will be discharged fromthe anode and the cathode of the polymer electrolyte fuel cell due tothe degradation of the polymer electrolyte membrane.

Therefore, with a view to solving this problem, there are proposed afuel cell system which avoids a low humidification state in a shut downstate of the power generation operation to prevent degradation of apolymer electrolyte membrane and an operation method thereof (forexample, see Patent Document 1). Patent document 1 discloses that whenbringing the electric power generation operation of the fuel cell systemto a stop from a state in which it is being operated in the lowhumidification state, either the fuel gas dew point or the oxidizing gasdew point is controlled so that at least either the fuel gas dew pointor the oxidizing gas dew point is made to conform to the temperature ofthe polymer electrolyte fuel cell. Thereafter, the water content in thepolymer electrolyte membrane is controlled by cutting off the electricconnection between the polymer electrolyte fuel cell and an electricload.

On the other hand, there has been disclosed a configuration in which, incarbon monoxide removing device mounted in a fuel cell system, the airhumidified (oxidizing gas) is supplied to a fuel cell, and a part of thegas remaining unconsumed there and discharged as the cathode exhaustgas, is introduced into a selective oxidation reactor. Here, in apolymer electrolyte fuel cell, the reaction of oxygen and hydrogen inthe vicinity of the cathode forms water. Therefore, the oxygen contentof air discharged from the polymer electrolyte fuel cell lessens ascompared to its initial oxygen content, and the air becomes a watervapor-rich cathode exhaust gas whose nitrogen content has been increasedwith respect to the oxygen content thereof. Therefore, in this carbonmonoxide removing device, by the introduction of cathode exhaust gasinto the selective oxidation reactor, carbon monoxide and oxygen in areactant gas are attenuated simultaneously with increasing the heatcapacity of the reactant gas, whereby it becomes possible to avoid adrop in the reaction efficiency caused by rapid development of theselective oxidation reaction in the vicinity of the gas inlet port ofthe reactor (for example, see Patent Document 2).

Patent Document 1: WO 2007/046483 A1 Patent Document 2: Japanese PatentNo. 3732004 Publication DISCLOSURE OF THE INVENTION Problems to beSolved by the Invention

However, the proposal, as set forth above in Patent Document 1, requiresthat, when controlling the dew point of fuel gas, there should be achange in the operational condition of a reformer mounted in the fuelgas supplier. Alternatively, this proposal requires that, in order tocontrol the dew point of fuel gas, there should be a change in theoperational condition of a humidifier placed between the fuel gassupplier and the polymer electrolyte fuel cell.

However, in the case where the dew point of fuel gas is controlled bymaking a change in the operational condition of the reformer of the fuelgas supplier, more specifically, in the case where the dew point of fuelgas is increased by making a change in the S/C (steam/carbon) ratio, theheat of the reformer is lost to added water. Therefore, if the dew pointof fuel gas is controlled by making a change in the operationalcondition of the reformer, this causes a new problem that the reformingefficiency of the reformer will become lower.

Besides, in the case where the dew point of fuel gas is controlled bymaking a change in the operational condition of the humidifier, it isrequired that the moisture should be newly supplied to the humidifier inorder to obtain a supply gas higher in dew point than the exhaust gasdischarged from the polymer electrolyte fuel cell, because of theconfiguration of the humidifier by which moisture present in the exhaustgas discharged from the polymer electrolyte fuel cell is transferred tothe supply gas which is fed to the polymer electrolyte fuel cell.Therefore, if the dew point of fuel gas is controlled by making a changein the operational condition of the humidifier, this requires that awater passage for supplying the moisture to the humidifier should benewly provided, and it is therefore anticipated that the costs willincrease and the system will become complicated.

On the other hand, if, in the carbon monoxide removing device mounted inthe fuel cell system as set forth above in Patent Document 2, cathodeexhaust gas discharged from the fuel cell is introduced into theselective oxidation reactor, this means that moisture is given to thereactant gas in the selective oxidation reactor. However, thistechnology is intended to suppress the selective oxidation reaction fromrapid development by attenuating carbon monoxide and oxygen in thereactant gas while increasing the heat capacity of the reactant gas, andit is therefore presumed that this technology is intended mainly for useduring the electric power generation operation in which the fuel gas issupplied to the fuel cell. That is to say, the proposal as set forthabove in Patent document 2 is not intended for the humidification of thepolymer electrolyte membrane during the non electric power generationoperation of the fuel cell and no mention is made at all of this.

The present invention was made with a view to providing solutions to theabove-mentioned problems. Accordingly, an object of the presentinvention is to provide a fuel cell system with high durability, whichis capable of preventing degradation of a polymer electrolyte membraneduring transition of a polymer electrolyte fuel cell being operated in alow humidification state to an open circuit state by controlling a fuelgas dew point with a simple configuration, and an operation methodthereof.

Means for Overcoming the Problems

Through a series of their research efforts dedicated to accomplishingthe above-mentioned object, the inventors of the present patentapplication attained the present invention from their discovery of thefact that it is preferable, in a fuel cell system including a polymerelectrolyte fuel cell, to control and cause at least the dew point ofeither fuel gas or oxidizing gas supplied to the fuel cell to increasebefore cutting off the electrical connection of the polymer electrolytefuel cell with an electric load and, thereafter, to cut off the electricconnection between the polymer electrolyte fuel cell and the electricload.

That is to say, the present invention provides a fuel cell system andits operating system. More specifically, the fuel cell system accordingto the present invention comprises a fuel cell system comprising: a fuelcell configured to generate electric power using fuel gas containinghydrogen and oxidizing gas containing oxygen; a fuel gas supplierconfigured to supply the fuel gas to the fuel cell; an oxidizing gassupplier configured to supply the oxidizing gas to the fuel cell; and acontroller configured to at least control the fuel cell, the fuel gassupplier and the oxidizing gas supplier; wherein the fuel cell systemfurther comprises a moisture supply mechanism configured to supplymoisture to at least one of an anode and a cathode of the fuel cell; andwherein the controller is configured to control the moisture supplymechanism based on at least one of either a dew point of the fuel gas orinformation related to the dew point of the fuel gas and either a dewpoint of the oxidizing gas or information related to the dew point ofthe oxidizing gas so as to increase at least one of the dew point of thefuel gas and the dew point of the oxidizing gas, in order to supply themoisture to at least one of the anode and the cathode of the fuel cellbefore cutting off electric connection between the fuel cell and a load,and is configured to thereafter cut off the electric connection betweenthe fuel cell and the load.

In such a configuration, in the process of shutting down the electricpower generation operation of the fuel cell system, the moisture isproperly supplied to the fuel cell. This properly increases at least oneof the fuel gas dew point and the oxidizing gas dew point. Therefore,the moisture content of the polymer electrolyte membrane can berelatively and properly increased by a simple configuration whereby itbecomes possible to suppress the degradation of the polymer electrolytemembrane. This makes it possible to provide a fuel cell system havinghigh durability.

In this case, the fuel gas supplier comprises: a reformer configured togenerate a fuel gas containing carbon monoxide using a raw materialthrough a reforming reaction, a shift converter configured to decreasecarbon monoxide in the fuel gas generated in the reformer through ashift reaction, and a selective oxidation unit configured to furtherdecease the carbon monoxide in the fuel gas with its carbon monoxidedecreased in the shift converter through a selective oxidation reaction,wherein the moisture supply mechanism is a selective oxidation moisturesupply mechanism configured to supply moisture to the selectiveoxidation unit.

In such a configuration, the moisture is supplied by the selectiveoxidation moisture supply mechanism to the selective oxidation unit.This makes it possible to suppress the degradation of the polymerelectrolyte membrane in the fuel cell system provided with the fuel gassupplier.

In this case, wherein the selective oxidation unit comprises: aselective oxidation air supply path used for supplying selectiveoxidation air to the fuel gas with its carbon monoxide decreased in theshift converter; a mixing section configured to mix the selectiveoxidation air delivered through the selective oxidation air supply pathand the fuel gas with its carbon monoxide decreased in the shiftconverter; and a selective oxidation catalytic unit configured to, usinga mixture gas of the fuel gas and the selective oxidation air, obtainedby mixing in the mixing section, reduce the carbon monoxide in themixture gas through the selective oxidation reaction, wherein theselective oxidation moisture supply mechanism is configured to supplythe moisture to either the selective oxidation air supply path or to themixing section.

In such a configuration, the moisture is supplied to either theselective oxidation air supply path or the mixing section, therebymaking it possible to provide well mixing of fuel gas discharged fromthe shift converter with moisture. This makes it possible to properlyincrease the fuel gas dew point.

In this case, wherein the selective oxidation moisture supply mechanismincludes: a water tank configured to store water; a moisture supply pathfor providing communication between the water tank and the selectiveoxidation unit; and a moisture content regulation unit disposed in themoisture supply path.

In such a configuration, it becomes possible to construct a selectiveoxidation moisture supply mechanism using a relatively simpleconfiguration including a water tank, a moisture supply path and amoisture content regulation unit. This makes it possible to avoid thefuel cell system from becoming complicated in its configuration.

Besides, in the above-mentioned case, the fuel cell system furthercomprise a selective oxidation air supply unit configured to supplyselective oxidation air to the selective oxidation unit, and wherein theselective oxidation moisture supply mechanism includes: a cathodeoff-gas bypass path used for supplying cathode off-gas containing unusedoxidizing gas discharged from the fuel cell, to the selective oxidationunit; and a selective oxidation air regulation unit configured to supplyat least one of the cathode off-gas delivered through the cathodeoff-gas bypass path and the selective oxidation air supplied from theselective oxidation air supply unit, to the selective oxidation unit.

In such a configuration, during the shutdown operation of the fuel cell,the selective oxidation unit of the fuel gas supplier is supplied withcathode off-gas containing oxidizing gas discharged from the fuel cellwhile maintaining the electric power generation of the fuel cell,whereby the dew point of fuel gas which is supplied to the fuel cell canbe increased properly. In addition, the cathode off-gas contains water(water vapor) formed by electrochemical reaction of fuel gas andoxidizing gas in the fuel cell, and this water is utilized as the waterto increase the dew point of fuel gas in the selective oxidation unit,in other words the increase in the fuel gas dew point can beaccomplished by a simple configuration. As a result of this, themoisture content of the polymer electrolyte membrane of the fuel cellduring the non electric power generation can be increased relatively ascompared to during the electric power generation, thereby making itpossible to suppress the degradation of the polymer electrolytemembrane. This makes it possible to enhance the durability of the fuelcell system.

Besides, in the above-mentioned case, the fuel cell system furthercomprises: a selective oxidation air supply unit configured to supplyselective oxidation air to the selective oxidation unit, wherein thecontroller is configured to control at least one of theselective-oxidization air supply unit and the selective oxidationmoisture supply mechanism to make the temperature of the selectiveoxidation unit equal to or higher than a predetermined threshold, whencontrolling the selective oxidation moisture supply mechanism so thatthe moisture is supplied to the selective oxidation unit.

In such a configuration, when the selective oxidation moisture supplymechanism is controlled so that the moisture is supplied to theselective oxidation unit, the air is supplied to the selective oxidationunit, for example, from the selective oxidation air supply unit tothereby suppress the drop in the temperature of the selective oxidationunit, whereby it becomes possible to prevent the efficiency of cuttingdown the carbon monoxide from dropping. This makes it possible to ensurethe quality of fuel gas discharged from the selective oxidation unit.

Besides, in the above-mentioned case, the controller is configured toperform control to cause the selective oxidation unit to be filledtherein with the fuel gas, after cutting off the electric connectionbetween the fuel cell and the load.

In such a configuration, the inside of the selective oxidation unit isfilled up with fuel gas after the electric connection between the fuelcell and the load is cut off, whereby the moisture content within theselective oxidation unit can be reduced. In this way, by the drying ofthe inside of the selective oxidation unit and the removal of moisture,it becomes possible to suppress the degradation of the selectiveoxidation catalyst.

Besides, in the above-mentioned case, the fuel cell system furthercomprises a water tank for storing water; a second moisture supply pathfor providing communication between the water tank and at least one ofthe anode and the cathode of the fuel cell; and a second moisturecontent regulation unit disposed in the second moisture supply path,wherein the moisture supply mechanism is a fuel-cell moisture supplymechanism configured to supply the moisture from the water tank to atleast one of the anode and the cathode of the fuel cell.

In such a configuration, at least one of the anode and the cathode ofthe fuel cell is supplied with moisture directly from the water tank,whereby the degradation of the polymer electrolyte membrane can besuppressed even in a fuel cell system as a drive electric power source,for example, for use in an electric vehicle which is equipped with ahydrogen cylinder in place of a fuel gas supplier.

In this case, the fuel cell system further comprises: a temperaturecontrol device configured to control a temperature of the fuel cell; anannular heating medium path used for circulating a heating mediumbetween the temperature control device and the fuel cell to transferheat from the fuel cell to the temperature control device; and a heatexchanger; wherein the heat exchanger is configured to exchange heatbetween the annular heating medium path and the second moisture supplypath.

In such a configuration, heat is exchanged by the heat exchanger betweenthe heating medium path and the second moisture supply path, therebymaking it possible to effectively use the heat of the fuel cell duringthe shutdown operation of the fuel cell system. Besides, whileincreasing at least one of the fuel gas dew point and the oxidizing gasdew point, the drop in the temperature of the fuel cell is expeditedand, therefore, it becomes possible to reduce the standby time until theelectric connection between the fuel cell and the electric load is cutoff.

Besides, in this case, the fuel gas supplier comprises: a reformerconfigured to generate a fuel gas containing carbon monoxide using a rawmaterial through a reforming reaction; and a heat exchanger; wherein theheat exchanger is configured to exchange heat between the reformer andthe second moisture supply path.

In such a configuration, heat is exchanged by the exchanger between thereformer and the second moisture supply path, thereby making it possibleto effectively utilize the heat of the reformer during the shutdownoperation of the fuel cell system.

Besides, the drop in the temperature of the reformer is expedited,thereby making it possible to reduce the standby time until the electricconnection between the fuel cell and the electric load is cut off.

Besides, in the above-mentioned case, the controller is configured toperform control such that the dew point of the fuel gas and the dewpoint of the oxidizing gas are lower than the temperature of the fuelcell, during an electric power generation operation of the fuel cellsystem.

In such a configuration, the fuel cell of the fuel cell system generateselectric power in the low humidification state and, therefore, theenergy conversion efficiency becomes improved. Besides, the polymerelectrolyte membrane of the fuel cell which generates electric power inthe low humidification state will be suitably moisturized during the nonelectric power generation.

Besides, in the above-mentioned case, the controller is configured tocontrol the moisture supply mechanism based on at least one of eitherthe dew point of the fuel gas or the information related to the dewpoint of the fuel gas and either the dew point of the oxidizing gas orthe information related to the dew point of the oxidizing gas so as toincrease at least one of the dew point of the fuel gas and the dew pointof the oxidizing gas to cause the temperature of the fuel cell toconform to the dew point of at least one of the fuel gas and theoxidizing gas, in order to supply the moisture to at least one of theanode and the cathode of the fuel cell before cutting off the electricconnection between the fuel cell and the load, and is configured tothereafter cut off the electric connection between the fuel cell and theload.

Alternatively, in the above-mentioned case, wherein the controller isconfigured to control the moisture supply mechanism based on at leastone of either the dew point of the fuel gas or the information relatedto the dew point of the fuel gas and either the dew point of theoxidizing gas or the information related to the dew point of theoxidizing gas so as to increase at least one of the dew point of thefuel gas and the dew point of the oxidizing gas, in order to supply themoisture to at least one of the anode and the cathode of the fuel cellbefore cutting off the electric connection between the fuel cell and theload; and wherein the controller is configured to perform control suchthat the dew point of at least one of the fuel gas and the oxidizing gasis equal to or higher than the temperature of the fuel cell, whencutting off electric connection between the fuel cell and the load.

In such a configuration, either the dew point of fuel gas or the dewpoint of oxidizing gas within the fuel cell becomes higher than or equalto the temperature of the fuel cell during the shutdown operation andthe shutdown of the fuel cell system, whereby the polymer electrolytemembrane of the fuel cell is well humidified by moisture contained in atleast one of the fuel gas and the oxidizing gas. Therefore, it becomespossible to contribute to the improvement in the durability of the fuelcell by suppressing the degradation of the polymer electrolyte membrane.

ADVANTAGEOUS EFFECTS OF THE INVENTION

In accordance with the configurations of the fuel cell systems and theoperation methods thereof according to the present invention, it becomespossible to provide a fuel cell system having high durability andcapable of preventing, by controlling the dew point of fuel gas using asimple configuration, the polymer electrolyte membrane from undergoingdegradation when the polymer electrolyte fuel cell, which is operated ina low humidification state, changes its state to the open circuit state.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view schematically illustrating a crosssectional structure of a fuel cell mounted in a fuel cell systemaccording to a first embodiment of the present invention.

FIG. 2 is a block diagram schematically illustrating a firstconfiguration of the fuel cell system according to the first embodimentof the present invention.

FIG. 3 is a block diagram schematically illustrating a secondconfiguration of the fuel cell system according to the first embodimentof the present invention.

FIG. 4 is a flow chart schematically representing a first characteristicoperation of the fuel cell system according to the first embodiment ofthe present invention.

FIG. 5 is a time chart schematically representing how the fuel celltemperature Tcell, the fuel gas dew point Tda, the oxidizing gas dewpoint Tdc, the selective oxidation unit temperature Tprox and the fuelcell output voltage Vfc each vary in the first characteristic operationof the fuel cell system according to the first embodiment of the presentinvention.

FIG. 6 is a flow chart schematically representing a secondcharacteristic operation of the fuel cell system according to the firstembodiment of the present invention.

FIG. 7 is a time chart schematically representing how the fuel celltemperature Tcell, the fuel gas dew point Tda, the oxidizing gas dewpoint Tdc, the selective oxidation unit temperature Tprox and the fuelcell output voltage Vfc each vary in the second characteristic operationof the fuel cell system according to the first embodiment of the presentinvention.

FIG. 8, comprised of a perspective view and a top plan view,schematically depicts a concrete configuration of a selective oxidationunit according to the first embodiment of the present invention.

FIG. 9 is a block diagram schematically illustrating a firstconfiguration of a fuel cell system according to a second embodiment ofthe present invention.

FIG. 10 is a block diagram schematically illustrating a secondconfiguration of the fuel cell system according to the second embodimentof the present invention.

FIG. 11 is a flow chart schematically representing a characteristicoperation of the fuel cell system according to the second embodiment ofthe present invention.

FIG. 12 is a time chart schematically representing how the fuel celltemperature Tcell, the fuel gas dew point Tda, the oxidizing gas dewpoint Tdc, the oxidation unit temperature Tprox and the fuel cell outputvoltage Vfc each vary in the characteristic operation of the fuel cellsystem according to the second embodiment of the present invention.

FIG. 13 is a block diagram schematically illustrating a thirdconfiguration of the fuel cell system according to the second embodimentof the present invention.

FIG. 14 is a block diagram schematically illustrating a firstconfiguration of a fuel cell system according to a third embodiment ofthe present invention.

FIG. 15 is a block diagram schematically illustrating a secondconfiguration of the fuel cell system according to the third embodimentof the present invention.

FIG. 16 is a block diagram schematically illustrating a firstconfiguration of a fuel cell system according to a fourth embodiment ofthe present invention.

FIG. 17 is a block diagram schematically illustrating a secondconfiguration of the fuel cell system according to the fourth embodimentof the present invention.

REFERENCE NUMERALS IN THE DRAWINGS

-   1 polymer electrolyte membrane-   2 a catalytic reaction layer (on anode side)-   2 c catalytic reaction layer (on cathode side)-   3 a gas diffusion layer (on anode side)-   3 c gas diffusion layer (on cathode side)-   4 a anode-   4 c cathode-   5 MEA (membrane/electrode assembly)-   6 a fuel gas passage-   6 c oxidizing gas passage-   7 a separator (one anode side)-   7 c separator (on the cathode side)-   8 a, 8 c cooling water passage-   9 a, 9 c gasket-   10 gasket-   11 fuel cell (polymer electrolyte fuel cell)-   12 fuel gas supply path-   13 oxidizing gas supply path-   14 anode off-gas discharge path-   15 cathode off-gas discharge path-   16 fuel gas supplier-   17 oxidizing gas supplier-   18 humidifier-   19 temperature control device-   20 controller-   21 a, 21 c dew point sensor-   22 temperature sensor-   23 reformer-   24 shift converter-   25 selective oxidation unit-   26 on-off valve (air supply mechanism)-   27 on-off valve (water supply mechanism)-   28 water supply pump-   29 water tank-   30 hydrogen cylinder-   31 inner tube-   32 outer tube-   33 air supply unit-   34 shift gas supply unit-   35, 36 mixing section-   27 selective oxidation catalytic unit-   38, 39, 40 aperture-   41 on-off valve (hydrogen supply mechanism)-   42 heat exchanger-   43 first three-way valve (oxidizing gas supply path switch    mechanism)-   44 second three-way valve (oxidizing gas discharge switch mechanism)-   45 third three-way valve-   46 on-off valve-   47 bypass path-   48 oxidation-reaction oxidizing gas supply path-   49 burner-   50 fuel gas outlet port-   51 oxidizing gas outlet port-   52 selective gas supply unit-   53 circulation path-   54 cathode off-gas supply path-   55 variable orifice-   56 pump-   57 selective gas supply unit-   58 fuel gas supply line-   59 oxidizing gas supply line-   60 fuel gas discharge line-   61 oxidizing gas discharge line-   62 output controller-   100 a-100 i fuel cell system-   101 fuel cell-   102 unit cell (cell)-   103 selective oxidation unit

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the best modes for carrying out the present invention willbe described in detail with reference to the accompanying drawings. Inaddition, it should be noted that, in the following description, thesame or equivalent elements referred to throughout all the drawings areassigned the same reference numerals and their unnecessary repetition ofthe description will be omitted. Besides, in the following description,the polymer electrolyte fuel cell is referred to just as the “fuelcell”. And, the fuel cell system including such a fuel cell is referredto just as the “fuel cell system”. Furthermore, in the followingdescription, the membrane/electrode assembly is referred to just as the“MEA”.

Besides, in the specification, the term “moisture” means liquid water,gaseous water (that is, water vapor), liquid water-gaseous watermixture, or other like form of water. In addition, from the point ofview of accomplishing the feature of the present invention that the dewpoint of fuel gas and the dew point of oxidizing gas are increased, itis possible that, instead of using liquid water, gaseous water, or amixture of the two, solid water (i.e., ice) can be used as the“moisture” if the fuel cell system is configured suitably thereto(although not exemplarily described in the following description).Alternatively, substances that containing water molecules or substancescapable of providing water molecules by, for example, chemical reactionmay be used as the “moisture” if the fuel cell system is configuredsuitably thereto.

First Embodiment

With reference first to FIGS. 1 and 2, a configuration of a fuel cellsystem according to a first embodiment of the present invention will bedescribed.

FIG. 1 is a cross sectional view schematically illustrating a crosssectional structure of a fuel cell provided in the fuel cell systemaccording to the first embodiment of the present invention. It should benoted that FIG. 1 selectively diagrams only a principal section of thefuel cell necessary for providing a clear understanding of the basicconfiguration of the fuel cell.

As illustrated in FIG. 1, in a fuel cell 101 according to the firstembodiment, an MEA 5 is sandwiched between a separator 7 a and aseparator 7 c, with a pair of gaskets 9 a and 9 c disposed around theouter edge thereof. This constitutes a unit cell 102 in the fuel cell101. In addition, in the following description, the term “unit cell”will be referred to as the “cell” for the sake of simplification. And aplurality of such cells 102 are serially stacked in layers on upon theother to form a fuel cell 101.

Describing more concretely, FIG. 1 shows that the MEA 5 is provided witha proton conductive polymer electrolyte membrane 1. The polymerelectrolyte membrane 1 selectively transports protons, when being in themoisture state. The proton transport capacity of the polymer electrolytemembrane 1 is achieved as follows. That is, fixed charges fixed to thepolymer electrolyte membrane 1 in the moisture state areelectrolytically dissociated, and hydrogen functioning as a counterionfor the fixed charges is ionized to become movable. It is preferred thata perfluorocarbon sulfonic acid membrane, such as NAFION (registeredtrademark) of DuPont may be used as the polymer electrolyte membrane 1.As shown in FIG. 1, a catalytic reaction layer 2 a and a catalyticreaction layer 2 c, each formed mainly of carbon powder supporting ametallic catalyst of the platinum-based, are attached respectively tothe centers of both the surfaces of the polymer electrolyte membrane 1so that these layers are opposite to one another. In the catalyticreaction layer 2 a of the catalytic reaction layers 2 a and 2 c,hydrogen, derived from fuel gas from the fuel gas supplier (not shown inFIG. 1), is converted into electrons and protons, as represented by thefollowing chemical formula (1). Here, the electrons generated in thecatalytic reaction layer 2 a will reach, by way of an electric load (notshown in FIG. 1) connected to the fuel cell system, the catalyticreaction layer 2 c. Besides, the protons generated in the catalyticreaction layer 2 a will reach, through the polymer electrolyte membrane1, the catalytic reaction layer 2 c. Meanwhile, in the catalyticreaction layer 2 c of the fuel cell 101, the incoming electrons by wayof the electric load, the protons permeating through the polymerelectrolyte membrane 1 and the oxygen derived from oxidizing gassupplied from the oxidizing gas supplier (not shown in FIG. 1) are usedto form water as expressed by the following chemical formula (2). Withthe development of this series of chemical reactions, the fuel cell 101outputs electric power and, in addition, generates heat.

H₂→2H⁺+2e ⁻  chemical formula (1)

(1/2)O₂+2H⁺+2e ⁻→H₂O  chemical formula (2)

In addition, as shown in FIG. 1, a pair of a gas diffusion layer 3 a anda gas diffusion layer 3 c are disposed respectively on the surfaces ofthe catalytic reaction layers 2 a and 2 c which surfaces are not incontact with the polymer electrolyte membrane 1 so that these layers areopposite to one another. The gas diffusion layers 3 a and 3 c eachcombine permeability to fuel gas and oxidizing gas and conductivity, andare attached respectively to the surfaces of the catalytic reactionlayers 2 a and 2 c so that these layers are electrically connectedthereto.

And, in the fuel cell 101, the catalytic reaction layer 2 a and the gasdiffusion layer 3 a together constitute an anode 4 a. Besides, in asimilar way, in the fuel cell 101, the catalytic reaction layer 2 c andthe gas diffusion layer 3 c together constitute a cathode 4 c. And, inthe fuel cell 101, the polymer electrolyte membrane 1, the anode 4 a andthe cathode 4 c together constitute an MEA 5.

On the other hand, as illustrated in FIG. 1, the polymer electrolytemembrane 1 of the MEA 5 is sandwiched between a gasket 9 a and a gasket9 c each having electric insulating property and, further, these gaskets9 a and 9 c are sandwiched between a separator 7 a and a separator 7 ceach having electric conductive property. This configuration constitutesa cell 102 in the fuel cell 101. Here, in the cell 102, a fuel gaspassage 6 a is formed, in the surface of the cell 102 in contact withthe gas diffusion layer 3 a of the separator 7 a, in the shape of arecess. The fuel gas passage 6 a is used for supplying, to the gasdiffusion layer 3 a of the MEA 5, of fuel gas supplied from a fuel gassupplier and for discharging to outside the cell 102, of surplus fuelgas (anode off-gas). Besides, in this cell 102, an oxidizing gas passage6 c is formed, in the surface of the cell 102 in contact with the gasdiffusion layer 3 c of the separator 7 c, in the shape of a recess. Theoxidizing gas passage 6 c is used for supplying, to the gas diffusionlayer 3 c of the MEA 5, of oxidizing gas supplied from an oxidizing gassupplier and for discharging, to outside the cell 102, of gas generatedby catalytic reaction and surplus oxidizing gas (cathode off-gas). Inaddition, the separator 7 a and the gas diffusion layer 3 a areelectrically connected to each other, and the separator 7 c and the gasdiffusion layer 3 c are also electrically connected to each other.

And, as shown in FIG. 1, a plurality of cells 102 are electricallyserially stacked in layers on upon the other to thereby constitute afuel cell 101. In the fuel cell 101, a plurality of cells 102 areelectrically serially stacked in layers one upon the other so that aseparator 7 a of one cell 102 and a separator 7 c of the other cell 102are mutually electrically connected together to obtain a desired outputvoltage. Here, in the fuel cell 101, recesses opposite to each other areformed respectively in a surface of the separator 7 a in contact withthe separator 7 c and in a surface of the separator 7 c in contact withthe separator 7 a, whereby a cooling water passage 8 a and a coolingwater passage 8 c are provided. Besides, a gasket 10 is disposed betweenthe separator 7 a and the separator 7 c for preventing leakage ofcooling water distributed through the cooling water passages 8 a and 8c. In addition, the cooling water passages 8 a and 8 are supplied withcooling water from a cooling water supplier (not shown in FIG. 1), andthe cooling water thus supplied is used to cool the fuel cell 101 whichgenerates heat during its electric power generation operation. And, heatenergy recovered from the fuel cell 101 by cooling water is used forsupplying hot water.

FIG. 2 is a block diagram schematically illustrating a firstconfiguration of the fuel cell system according to the first embodimentof the present invention. It should be noted that FIG. 2 selectivelydiagrams only constituent components necessary for describing thepresent invention, and the diagrammatic representation of the otherremaining constituent components is omitted.

Referring to FIG. 2, there is shown a fuel cell system 100 a accordingto the first embodiment of the present invention that has basically thesame configuration as the configuration of a general fuel cell system ofthe conventional type.

More specifically, as shown in FIG. 2, the fuel cell system 100 aaccording to the first embodiment of the present invention is provided,as a main part of its power generating section, with a fuel cell 11which generates electric power when supplied with fuel gas and oxidizinggas. Besides, the fuel cell system 100 a includes a fuel gas supplier 16for supplying fuel gas to the fuel cell 11, an oxidizing gas supplier 17for supplying oxidizing gas to the fuel cell 11, a humidifier 18 forhumidifying oxidizing gas supplied from the oxidizing gas supplier 17 tothe fuel cell 11 along the way thereto, a dew point sensor 21 a fordetecting the dew point of fuel gas, and a dew point sensor 21 c fordetecting the dew point of oxidizing gas. Besides, the fuel cell system100 a further includes a temperature control device 19 for controlling,by means of cooling medium, the temperature of the fuel cell 11 when itgenerates electric power upon the supply of fuel gas and oxidizing gas,and a temperature sensor 22 for detecting the temperature of coolingmedium circulating between the temperature control device 19 and thefuel cell 11. Furthermore, the fuel cell system 100 a includes acontroller 20 which properly controls the operations of the fuel cell11, the fuel gas supplier 16, the oxidizing gas supplier 17 and thetemperature control device 19.

Here, as shown in FIG. 2, connected to the fuel cell 11 according to thepresent embodiment are: a fuel gas supply path 12 through which fuel gasis supplied, an oxidizing gas supply path 13 through which oxidizing gasis supplied, an anode off-gas discharge path 14 through which surplusfuel gas is discharged, and a cathode off-gas discharge path 15 throughwhich surplus oxidizing gas is supplied. And, during the electric powergeneration operation of the fuel cell system 100 a, the dew point offuel gas generated by the fuel gas supplier 16 is detected by the dewpoint sensor 21 a and, thereafter, the fuel gas is delivered to the fuelgas supply path 12 connected to the fuel cell 11. Besides, surplus fuelgas in the fuel cell 11 is discharged out from the anode off-gasdischarge path 14 connected to the fuel cell 11. Meanwhile, oxidizinggas from the oxidizing gas supplier 17 is humidified in the humidifier18 and, after its dew point is detected by the dew point sensor 21 c, isdelivered to the oxidizing gas supply path 13 connected to the fuel cell11. Besides, surplus oxidizing gas in the fuel cell 11 is discharged outfrom the cathode off-gas discharge path 15 connected to the fuel cell11.

In the present embodiment, the fuel gas supplier 16 uses a raw material(such as town gas, propane gas, or other like gas) to generate, in areformer 23, a hydrogen-containing gas rich in hydrogen (hereinafter,referred to as the “fuel gas” for the sake of simplicity) bysteam-reforming reaction. Here, the fuel gas generated in the reformer23 is rich in hydrogen but contains much carbon monoxide harmful to thefuel cell 11. Therefore, in the present embodiment, the shift converter24 of the fuel gas supplier 16 expedites a predetermined shift reaction,thereby reducing the content of carbon monoxide present in the fuel gasgenerated in the reformer 23. And, the fuel gas, whose carbon monoxidehas been reduced in the shift converter 24, is then supplied to theselective oxidation unit 25. The fuel gas thus supplied is subjected tocarbon monoxide removal by burning in the selective oxidation unit 25 towhich oxidizing gas for oxidation reaction is supplied, and theresulting fuel gas after proper reduction in the carbon monoxide isdelivered, as the fuel gas, to the fuel cell 11 from the fuel gassupplier 16.

Here, as shown in FIG. 2, the fuel cell system 100 a according to thepresent embodiment is configured such that the oxidizing gas foroxidation reaction is supplied, via an on-off valve 26, to the selectiveoxidation unit 25. Besides, as shown in FIG. 2, the fuel cell system 100a is configured such that water stored in a water tank 29 is supplied,through a water supply pump 28 (which is a moisture content regulator)and then through an on-off valve 27, to the selective oxidation unit 25.In addition, as the water tank 29, tanks generally used in fuel cellsystems (such as condensed water tanks for storing condensed waterseparated from anode off-gas and cathode off-gas, reform water tanks forstoring reform water for the steam reforming reaction in the reformer,cooling water tanks for storing cooling water for the cooling of thefuel cell, and other similar tanks (not shown in FIG. 2)) may berecited.

In this way, in the present embodiment, the fuel gas is humidified inthe steam reforming reaction and is supplied, in a water vaporcontaining state, to the fuel cell 11. This maintains the polymerelectrolyte membrane (not shown in FIG. 2) of the fuel cell 11 in apredetermined wet state.

On the other hand, in the present embodiment, the oxidizing gas supplier17 takes in air as the oxidizing gas for oxidation reaction from theatmosphere with the aid of, for example, a sirocco fan. And, theoxidizing gas supplier 17 delivers the air thus taken therein to thehumidifier 18. Then, the humidifier 18 humidifies the air suppliedthereto and, thereafter, delivers the humidified air to the fuel cell11.

In the present embodiment, the dew point sensor 21 a detects the dewpoint of fuel gas supplied to the fuel cell 11 from the fuel gassupplier 16. Besides, the dew point sensor 21 c detects the dew point ofoxidizing gas supplied via the humidifier 18 to the fuel cell 11 fromthe oxidizing gas supplier 17. Here, in the present embodiment, the fuelgas dew point and the oxidizing gas dew point detected respectively bythe dew point sensor 21 a and by the dew point sensor 21 c are regardedrespectively as the fuel gas dew point Tda and as the oxidizing gas dewpoint Tdc in the inside of the fuel cell 11. In addition, as the dewpoint sensors 21 a and 21 c, any type of dew point sensor may beemployed as long as it has both durability against gases such as thefuel gas and the oxidizing gas and temperature durability. Besides, thefuel gas dew point Tda is dependent on the performance of the fuel gassupplier 16 and the oxidizing gas dew point Tdc is dependent on theperformance of the humidifier 18. Therefore, the configuration may bethat the dew point calculated based on the operational conditions of thefuel gas supplier 16 (parameters such as the flow rate of raw material,the amount of reform water, the reforming temperature et cetera) servesas the fuel gas dew point Tda while either the dew point calculatedbased on the operational conditions of the humidifier or the temperatureof the humidifier 18 serves as the oxidizing gas dew point Tdc. In otherwords, in the present embodiment, it is possible to employ, instead ofthe configuration that employs the dew point sensors 21 a and 21 c, aconfiguration in which the fuel gas dew point and the oxidizing gas dewpoint are determined by means of existing control elements for drivingthe fuel gas supplier 16. Even when employing such configuration, it isstill possible to properly determine the amounts of water and air thatare supplied to the selective oxidation unit 25, as will be describedlater.

On the other hand, in the present embodiment, the temperature controldevice 19 is made up of, for example, a circulation pump for circulatingcooling medium and a heat radiator (such as a cooling fin, a heatexchanger et cetera) for causing cooling medium in circulation torelease heat therefrom. By the temperature control device 19, thecooling medium is supplied to the fuel cell 11, and the cooling medium,the temperature of which has been increased by heating by heat generatedassociated with the generation of electric power, is recovered from thefuel cell 11. Then the temperature control device 19 cools thetemperature-increased cooling medium and then supplies thetemperature-decreased cooling medium to the fuel cell 11 again.Alternatively, the temperature control device 19 causes, by making achange in at least one of the conditions, i.e., the flow rate and thetemperature of cooling medium, the temperature of the fuel cell 11 todecrease. For example, it becomes possible to cool down the temperatureof the fuel cell 11 by increasing the flow rate of cooling medium in theinside of the temperature control device 19. As a result of this, thetemperature control device 19 maintains the temperature of the fuel cell11 at a constant level of temperature. In addition, heat energyrecovered from the fuel cell 11 by the cooling medium is used forsupplying hot water or other like application.

Here, the temperature sensor 22, shown in FIG. 2, detects thetemperature of cooling medium discharged to the temperature controldevice 19 from the fuel cell 11. In the present embodiment, thetemperature of cooling medium detected by the temperature sensor 22 isregarded as the temperature of the fuel cell 11, Tcell. In addition, thetemperature, Tcell, of the fuel cell 11 is the highest of thetemperatures of the fuel cell 11. It is conceivable to use, as such adetection method, a method which measures the temperature of coolingmedium supplied to the fuel cell 11, a method which directly measures,by means of a thermocouple, the temperature of a separator (not shown inFIG. 2) which composes the fuel cell 11, a method which measures thetemperature of cooling medium discharged from the fuel cell 11, or otherlike method. On the other hand, the portion that has the highest of thetemperatures of the fuel cell 11 is supposed to be an outlet portportion from which cooling medium fed from the inlet port will exit.Therefore, in the present embodiment, the configuration employed is thatthe temperature of cooling medium discharged to the temperature controldevice 19 from the fuel cell 11 is detected by the temperature sensor22.

In addition, in the present embodiment, the low humidification state maybe, for example, an operational condition in which: (the temperature atthe cooling-medium inlet port portion of the fuel cell 11)≦Tda<(thetemperature at the cooling-medium outlet port portion of the fuel cell11, Tcell) and, in addition, (the temperature at the cooling-mediuminlet port portion of the fuel cell 11)≦Tdc<(the temperature at thecooling-medium outlet port portion of the fuel cell 11, Tcell). Alsoeven in this case where at least a part of the inside of the fuel cell11 is being in the low humidification state, it is still possible toachieve the advantageous effects of the present invention.

Besides, in the present embodiment, the low humidification state may be,for example, an operational condition in which: Tda<(the temperature atthe cooling-medium inlet port portion of the fuel cell 11) and, inaddition, Tdc<(the temperature at the cooling-medium inlet port portionof the fuel cell 11). In this case, almost the entire area of the insideof the fuel cell 11 is placed in the low humidification state, wherebythe advantageous effects of the present invention can be accomplishedmore significantly.

That is, in the present embodiment, the low humidification state may be,for example, an operational condition in which: (the temperature at thecooling-medium inlet port portion of the fuel cell 11)≦Tda<(thetemperature at the cooling-medium inlet port portion of the fuel cell11, Tcell) and, in addition, Tdc<(the temperature at the cooling-mediuminlet port portion of the fuel cell 11). Alternatively, the lowhumidification state may be, for example, an operational condition inwhich: Tda<(the temperature at the cooling-medium inlet port portion ofthe fuel cell 11)≦Tdc<(the temperature at the cooling-medium outlet portportion of the fuel cell 11, Tcell). Also even in this case where atleast a part of the inside of the fuel cell 11 is being in the lowhumidification state, it is still possible to achieve the advantageouseffects of the present invention.

On the other hand, in the present embodiment, the controller 20 controlsthe operations of at least the fuel cell 11, the fuel gas supplier 16,the oxidizing gas supplier 17 and the temperature control device 19. Thecontroller 20 includes, for example, an MPU (microprocessor unit) and amemory and properly controls, based on data such as programs, parameterset cetera prestored in the memory, the operations of at least the fuelcell 11, the fuel gas supplier 16, the oxidizing gas supplier 17 and thetemperature control device 19. Here, in the present embodiment, the term“controller” means not only a single controller, but it also means acontroller group composed of a plurality of controllers. Accordingly,the controller 20 may be made up of a single controller or may becomposed of a plurality of controllers dispersedly configured forproviding cooperative control.

Next, referring to FIG. 3, a modification of the fuel cell systemaccording to the first embodiment of the present invention will bedescribed.

FIG. 3 is a block diagram schematically illustrating a secondconfiguration of the fuel cell system according to the first embodimentof the present invention.

FIG. 3 shows a fuel cell system 100 b according to the first embodimentof the present invention which has basically the same configuration asthe configuration of the fuel cell system 100 a (FIG. 2). That is, thefuel cell system 100 b includes a fuel gas supplier 16, an oxidizing gassupplier 17, a humidifier 18, a dew point sensor 21 a, a dew pointsensor 21 c, a fuel cell 11, a temperature control device 19 and acontroller 20. And, the fuel cell system 100 b is configured such thatthe fuel gas is supplied via the dew point sensor 21 a to the fuel cell11 from the fuel gas supplier 16 while the oxidizing gas is supplied,through the humidifier 18 and then through the dew point sensor 21 c, tothe fuel cell 11 from the oxidizing gas supplier 17. Besides, the fuelcell system 100 b is configured such that the temperature of the fuelcell 11 is controlled by the temperature control device 19 while theoperations of the fuel cell system 100 b are properly controlled by thecontroller 20.

On the other hand, unlike the configuration of the fuel cell system 100a in which the water is supplied, through the water supply pump 28 andthrough the on-off valve 27, to the selective oxidation unit 25 of thefuel gas supplier 16 from the water tank 29, the fuel cell system 100 baccording to the present embodiment is configured such that the water issupplied, through the water supply pump 28 and then through the on-offvalve 27, to the connecting piping of the shift converter 24 and theselective oxidation unit 25 from the water tank 29, as shown in FIG. 3.In addition, for the rest, the configuration is the same as in the fuelcell system 100 a shown in FIG. 2.

Next, a description will be made with respect to characteristicoperations of the fuel cell system according to the first embodiment ofthe present invention.

The operation of the fuel cell system 100 a according to the presentembodiment is the same as that of the conventional fuel cell system,with the exception that, prior to cutting off the electrical connectionof the fuel cell 11, which is operated in a low humidification state,with the electric load (that is, before the fuel cell 11 is placed inthe open circuit state), water is added to the selective oxidation unit25 of the fuel gas supplier 16 to thereby properly control the dew pointof fuel gas so that the polymer electrolyte membrane mounted in the fuelcell 11 is suitably humidified. Therefore, hereinafter, only thecharacteristic operations of the fuel cell system according to thepresent embodiment will be described in detail.

FIG. 4 is a flow chart schematically representing a first characteristicoperation of the fuel cell system according to the first embodiment ofthe present invention. It should be noted that FIG. 4 selectivelyrepresents only steps necessary for describing the present invention,and the diagrammatic representation of the other remaining steps isomitted.

Besides, FIG. 5 is a time chart schematically representing how the fuelcell temperature Tcell, the fuel gas dew point Tda, the oxidizing gasdew point Tdc, the selective oxidation unit temperature Tprox and thefuel cell output voltage Vfc each vary with time in the firstcharacteristic operation of the fuel cell system according to the firstembodiment of the present invention. It should be noted that FIG. 5selectively represents only operations necessary for describing thepresent invention, and the diagrammatic representation of the otherremaining operations is omitted.

As shown in FIGS. 4 and 5, in the present embodiment, in the fuel cellsystems 100 a and 100 b, the fuel cell 11 is being operated in a lowhumidification operation mode that meets both the interrelations:Tcell>Tda and Tcell>Tdc. (State 1 of FIG. 5) When bringing the electricpower generation operation of the fuel cell system 100 a, 100 b to astop, the controller 20 controls the on-off valve 27 (water supplymechanism) shown in FIGS. 2 and 3 to add water to the selectiveoxidation unit 25 (Step S1 of FIG. 4; Operation 1 of FIG. 5) to increasethe fuel gas dew point Tda until the fuel gas dew point Tda conforms tothe temperature Tcell of the fuel cell 11 (Step 2 of FIG. 4; State 2 ofFIG. 5). Here, in State 2 shown in FIG. 5, the controller 20 providescontrol that causes the fuel cell 11 to continue electric discharge,without cutting off the electric connection between the fuel cell 11 andthe electric load. In addition, as shown in FIG. 5, in State 2, theoxidizing gas dew point Tdc and the fuel cell temperature Tcell remainalmost unchanged, but the fuel gas dew point Tda increases with time andthe temperature, Tprox, of the selective oxidation unit 25 decreaseswith time because the selective oxidation unit 25 is supplied withwater. A more concrete description will be made with respect to a mannerfor compensating the drop in the temperature, Tprox, of the selectiveoxidation unit 25.

Next, based on output signals from the dew point sensor 21 a and thetemperature sensor 22, the controller 20 makes, while causing the fuelcell 11 to continue electric discharge, a decision of whether or not thefuel gas dew point Tda conforms to the temperature, Tcell, of the fuelcell 11 (Step S3).

More specifically, if the decision made in Step S3 indicates that thefuel gas dew point Tda does not conform to the temperature, Tcell, ofthe fuel cell 11 (“NO” in Step S3), the controller 20 continuesproviding control from Step S2 of FIG. 4 on until the time the fuel gasdew point Tda conforms to the temperature, Tcell, of the fuel cell 11,while the fuel cell 11 is left to continue electric discharge (State 2of FIG. 5).

On the other hand, if the decision made in Step S3 indicates that thefuel gas dew point Tda conforms to the temperature, Tcell, of the fuelcell 11 (“YES” in Step S3), the controller 20 controls the on-off valve27 (see FIGS. 2 and 3) to stop the supply of water to the selectiveoxidation unit 25 while causing the fuel cell 11 to continue electricdischarge (Step S4 of FIG. 4; Operation 2 of FIG. 5) whereby theincrease in the fuel gas dew point Tda is stopped (Step S5).

Then the controller 20 causes the fuel cell system 100 a, 100 b tomaintain its current operational state until the time the measured timeTm reaches a preset predetermined time Tpd (Step S6 of FIG. 4; State 3of FIG. 5). In State 5 shown in FIG. 5, the polymer electrolyte membraneof the fuel cell 11 is well humidified to a state that makes thedegradation of the polymer electrolyte membrane preventable by the useof moisture contained mainly in the fuel gas.

More specifically, in State 3 shown in FIG. 5, the controller 20 makes adecision of whether or not the measured time Tm reaches the presetpredetermined time Tpd (Step S6).

If the decision made in Step S6 indicates that the measured time Tm doesnot yet reach the preset predetermined time Tpd (“NO” in Step S6), thecontroller 20 causes the fuel cell system 100 a, 100 b to furthermaintain its current operational state until the time the measured timeTm reaches the preset predetermined time Tpd, while the fuel cell 11 isleft to continue electric discharge.

On the other hand, if the decision made in Step S6 indicates that themeasured time Tm reaches the preset predetermined time Tpd (“YES” inStep S6), the controller 20 cuts off the electric connection between thefuel cell 11 and the electric load (Operation 3 of FIG. 5) to therebycause the fuel cell 11 to stop electric discharge (Step S7). Then thecontroller 20 places the fuel cell 11 of the fuel cell system 100 a, 100b in the open circuit state (State 4 of FIG. 5).

Thereafter, the controller 20 stops the fuel gas supplier 16 and theoxidizing gas supplier 17 from operating. Then the controller 20 stopsall of the operations pertaining to the electric power generationoperation of the fuel cell system 100 a, 100 b.

Furthermore, after cutting off the electric connection between the fuelcell 11 and the electric load in Step S7, the controller 20 causes theselective oxidation unit 25 to be filled up with fuel gas whereby theselective oxidation unit 25 becomes dried. This operation makes itpossible to prevent the condensation of water in the selective oxidationunit 25 during the shutdown operation of the fuel cell system 100 a, 100b. As a result of which, it becomes possible to suppress the degradationof the selective oxidation catalyst mounted in the selective oxidationunit 25. Besides, this operation will not increase the start energy ofthe selective oxidation unit 25 whereby it becomes possible to provide afuel cell system of high efficiency.

Here, the above-described characteristic operation of the fuel cellsystem 100 a, 100 b is implemented by inputting predetermined programsto the memory of the controller 20.

In this way, in the present embodiment, when shutting down the electricpower generation operation of the fuel cell 11 from a state in which thefuel cell 11 is being operated in a low humidification state that meetsboth the interrelations: Tcell>Tda and Tcell>Tdc, the controller 20controls the fuel gas dew point Tda to increase by supplying water tothe selective oxidation unit 25, controls the fuel cell 11 to continueelectric discharge until the time the fuel gas dew point Tda conforms tothe temperature, Tcell, of the fuel cell 11, and controls the fuel cell11 to stop electric discharge to the electric load after the fuel gasdew point Tda conforms to the temperature, Tcell, of the fuel cell 11.This achieves the interrelation: Tcell≦Tda in the open circuit state inwhich the fuel cell 11 stops electric discharge to the electric load,and since the polymer electrolyte membrane is well humidified, thismakes it possible that the durability of the fuel cell 11 is ensuredsatisfactorily.

Here, with reference to FIG. 8, a description will be made of a concreteconfiguration of the selective oxidation unit 25 provided in the fuelcell system 100 a, 100 b according to the first embodiment of thepresent invention, and of the position for supplying water.

FIG. 8( a) is a perspective view schematically illustrating a concreteconfiguration of the selective oxidation unit provided in the fuel cellsystem according to the first embodiment of the present invention whileon the other hand FIG. 8( b) is a top plan view schematicallyillustrating a concrete configuration of the selective oxidation unitshown in FIG. 8( a) as viewed from thereabove. In addition, for the sakeof simplicity, FIG. 8( a) schematically illustrates a state that allowsthe internal portions of the selective oxidation unit to be visuallyseen by longitudinally cutting the external tube that composes theselective oxidation unit while on the other hand FIG. 8( b)schematically illustrates a state that allows internal portions of theselective oxidation unit to be visually seen by transversely cutting theexternal tube that composes the selective oxidation unit.

Referring now to FIGS. 8( a) and (8 b), there is shown a selectiveoxidation unit 103 according to the present embodiment. The selectiveoxidation unit 103 includes: an inner tube 31 constituting an inner walland an outer tube 32 constituting an outer wall which tubes are disposedconcentrically to one another; a shift gas supply unit 34 which issupplied with fuel gas (shift gas) from a shift converter (not shown);an air supply unit 33 for supplying air to the supplied fuel gas; mixingsections 35 and 36 for mixing of air supplied from the air supply unit33 and fuel gas supplied from the shift converter; and a selectiveoxidation catalytic unit 37 for reducing, by the use of a mixture gas ofthe fuel gas and the air discharged from the mixing section 36, theamount of carbon monoxide contained in the fuel gas by selectiveoxidation reaction. In addition, as shown in FIGS. 8( a) and (8 b), theselective oxidation unit 103 includes: an aperture 38 that allows fuelgas supplied to the shift gas supply unit 34 to travel to the mixingsection 35; a plurality of apertures 39 that allow fuel gas mixed withair in the mixing section 35 to further travel to the mixing section 36;and a plurality of apertures 40, disposed in the shape of a ring, thatallow fuel gas well mixed with air in the mixing section 36 to travel tothe selective oxidation catalytic unit 37.

And in the present embodiment, the water supplied to the selectiveoxidation unit 103 is supplied directly to the selective oxidationcatalytic unit 37 of the selective oxidation unit 103, or supplied tothe air supply unit 33 together with air. Alternatively, in the presentembodiment, the water supplied to the selective oxidation unit 103 issupplying directly to the mixing section 35 of the selective oxidationunit 103, or supplied directly to the mixing section 36 of the selectiveoxidation unit 103.

In this way, the water is supplied to the air supply unit 33, to themixing section 35, to the mixing section 36, or to the selectiveoxidation unit 37 whereby the water thus supplied is efficiently andevenly mixed with fuel gas by the flow of air or by the flow of fuelgas. This makes it possible to provide the supply, to the fuel cell 11of the fuel cell system 100 a, 100 b, of fuel gas which contains watervapor and whose dew point is properly controlled in an extremely simpleconfiguration.

Next, a concrete description will be made with respect to a manner thatcompensates the drop in the temperature, Tprox, of the selectiveoxidation unit 25 when increasing the fuel gas dew point Tda with time.

The configuration of a fuel cell system pertaining to a manner thatcompensates the drop in the temperature Tprox and the hardwareconfiguration of a fuel cell provided in the fuel cell system are thesame as that of the fuel cell system 100 a, 100 b formed in accordancewith the present embodiment (see FIGS. 1, 2, and 3) and as that of thefuel cell provided in the fuel cell system 100 a, 100 b. Accordingly, adescription of the configurations of the fuel cell system and the fuelcell provided in the fuel cell system is omitted.

And now, as has already been described above, the temperature, Tprox, ofthe selective oxidation unit 25 decreases with time because the water issupplied to the selective oxidation unit 25 when increasing the fuel gasdew point Tda. In this case, such decrease in the temperature, Tprox, ofthe selective oxidation unit 25 may cause the efficiency of selectiveoxidation that develops in the selective oxidation unit 25 to decrease.

Therefore, in the present embodiment, in order to suppress the drop withtime in the temperature, Tprox, of the selective oxidation unit 25, theair is supplied to the selective oxidation unit 25 when increasing thefuel gas dew point Tda. As a result of employing such configuration, thedrop in the efficiency of the selective oxidation reaction that developsin the selective oxidation unit 25 is suppressed.

Next, a description will be made in detail of a manner that compensatesthe drop in the temperature, Tprox, of the selective oxidation unit 25when increasing the fuel gas dew point Tda with time.

FIG. 6 is a flow chart schematically representing a secondcharacteristic operation of the fuel cell system according to the firstembodiment of the present invention. It should be noted that FIG. 6selectively represents only steps necessary for describing the presentinvention, and the diagrammatic representation of the other remainingsteps is omitted.

In addition, FIG. 7 is a time chart schematically representing how thefuel cell temperature Tcell, the fuel gas dew point Tda, the oxidizinggas dew point Tdc, the selective oxidation unit temperature Tprox, andthe fuel cell output voltage Vfc each vary in the second characteristicoperation of the fuel cell system according to the first embodiment ofthe present invention. It should be noted that FIG. 7 selectivelyrepresents only operations necessary for describing the presentinvention, and the diagrammatic representation of the other remainingoperations is omitted.

As shown in FIGS. 6 and 7, in the present embodiment, in the fuel cellsystems 100 a and 100 b, the fuel cell 11 is being operated in a lowhumidification operation mode that meets both the interrelations:Tcell>Tda and Tcell>Tdc (State 1 of FIG. 7). When shutting down theelectric power generation operation of the fuel cell system 100 a, 100b, the controller 20 controls the on-off valve 27 (water supplymechanism) to add water to the selective oxidation unit 25 (step S1 ofFIG. 6, operation 1 of FIG. 7) to increase the fuel gas dew point Tdauntil the fuel gas dew point Tda conforms to the temperature, Tcell ofthe fuel cell 11 (Step S2 of FIG. 6; State 2 of FIG. 7). Here, in State2 shown in FIG. 7, the controller 20 provides control that causes thefuel cell 11 to continue electric discharge, without cutting off theelectric connection between the fuel cell 11 and the electric load.

In this case, in State 2 as shown in FIG. 7, the oxidizing gas dew pointTdc and the fuel cell temperature Tcell remain almost unchanged, but thefuel gas dew point Tda increases with time while the temperature, Tprox,of the selective oxidation unit 25 decreases with time because the wateris supplied to the selective oxidation unit 25.

Therefore, in the present embodiment, the fuel cell 11 is made tocontinue electric discharge without cutting off the electric connectionbetween the fuel cell 11 and the electric load, while checking whetheror not the temperature, Tprox, of the selective oxidation unit 25 isdecreased down to a predetermined temperature threshold Tp or less bythe supply of water. And, if it is decided that the temperature, Tprox,of the selective oxidation unit 25 decreases to conform to thepredetermined temperature threshold Tp (“YES” in Step S3), thecontroller 20 controls the on-off valve 26 (an air supply mechanism)shown in FIGS. 2 and 3 so that air is further added to the selectiveoxidation unit 25 (Step S4 of FIG. 6; Operation 2 of FIG. 7), wherebythe temperature, Tprox, of the selective oxidation unit 25 is increased(State 3 of FIG. 7). In this State 3, in order that the temperature,Tprox, of the selective oxidation unit 25 is increased promptly, thecontroller 20 brings the supply of water to the selective oxidation unit25 to a temporary stop. Then, once it is confirmed that the temperature,Tprox, of the selective oxidation unit 25 is increased, the controller20 causes the supply of air to the selective oxidation unit 25 to stopwhile causing the supply of water to the selective oxidation unit 25 toresume (Step S1 of FIG. 6; Operation 3 of FIG. 7), whereby the fuel gasdew point Tda is again increased to conform to the temperature, Tcell,of the fuel cell 11 (Step S2 of FIG. 6; State 4 of FIG. 7).

Then if it is decided that the temperature, Tprox, of the selectiveoxidation unit 25 has not yet been decreased down to the predeterminedtemperature threshold Tp (“NO” in Step S3), the controller 20 furthermakes a decision, based on the output signals from the dew point sensor21 a and the temperature sensor 22, of whether or not the fuel gas dewpoint Tda conforms to the temperature, Tcell, of the fuel cell 11 whilecausing the fuel cell 11 to continue electric discharge (Step S5).

More specifically, if the decision made in this Step S5 indicates thatthe fuel gas dew point Tda does not conform to the temperature, Tcell,of the fuel cell 11 (“NO” in Step S5), the controller 20 continuesproviding control from Step S2 of FIG. 6 on (State 4 of FIG. 7) untilthe time the fuel gas dew point Tda conforms to the temperature, Tcell,of the fuel cell 11 while the fuel cell 11 is left to continue electricdischarge.

On the other hand, if the decision made in Step S5 indicates that thefuel gas dew point Tda conforms to the temperature, Tcell, of the fuelcell 11 (“YES” in Step S5), the controller 20 controls the on-off valve27 shown in FIGS. 2 and 3 so that the supply of water to the selectiveoxidation unit 25 is brought to a stop (Step S6 of FIG. 6; Operation 4of FIG. 7), while causing the fuel cell 11 to continue electricdischarge, whereby the increase in the fuel gas dew point Tda is ceased(Step S7).

Then the controller 20 provides control so that the operational state ofthe fuel cell system 100 a, 100 b is maintained as it is until the timethe measured time Tm reaches the preset predetermined time Tpd (Step S8of FIG. 6; State 5 of FIG. 7). In State 7 shown in FIG. 7, the polymerelectrolyte membrane of the fuel cell 11 is well humidified to a statethat makes the degradation of the polymer electrolyte membranepreventable by the use of moisture contained mainly in the fuel gas.

More specifically, in State 5 shown in FIG. 7, the controller 20 makes adecision of whether or not the measured time Tm has reached the presetpredetermined time Tpd (Step S8).

If the decision made in this Step S8 indicates that the measured time Tmhas not yet reached the preset predetermined time Tpd (“NO” in Step S8),the controller 20 provides control that the operational state of thefuel cell system 100 a, 100 b is further maintained as it is until thetime the measured time Tm reaches the preset predetermined time Tpd,while the fuel cell 11 is left to continue electric discharge.

On the other hand, if the decision made in Step S8 indicates that themeasured time Tm has reached the preset predetermined time Tpd (“YES” inStep S8), the controller 20 cuts off the electric connection between thefuel cell 11 and the electric load (Operation 5 of FIG. 7) to cause thefuel cell 11 to stop electric discharge (Step S9). Then the controller20 provides control that places the fuel cell 11 of the fuel cell system100 a, 100 b in the open circuit state (State 6 of FIG. 7).

Thereafter, the controller 20 stops the fuel gas supplier 16 and theoxidizing gas supplier 17 from operating. Then the controller 20 bringsall the operations relating to the electric power generation operationof the fuel cell system 100 a, 100 b to a stop. Besides, the controller20, after cutting off the electric connection between the fuel cell 11and the electric load in Step S9, causes the selective oxidation unit 25to be filled up with fuel gas, whereby the selective oxidation unit 25becomes dried.

In this way, in the present embodiment, when shutting down the electricpower generation operation of the fuel cell 11 from a state in which thefuel cell 11 is being operated in a low humidification state that meetsboth the interrelations: Tcell>Tda and Tcell>Tdc, the controller 20controls the fuel gas dew point Tda to increase by the supply of waterto the selective oxidation unit 25 while suppressing the drop in thetemperature, Tprox, of the selective oxidation unit 25. And thecontroller 20 controls the fuel cell 11 to continue to electricdischarge until the time the fuel gas dew point Tda conforms to thetemperature, Tcell, of the fuel cell 11, and controls the fuel cell 11to stop electric discharge to the electric load after the fuel gas dewpoint Tda conforms to the temperature, Tcell, of the fuel cell 11. Thisachieves the interrelation Tcell≦Tda in the open circuit state in whichthe fuel cell 11 stops electric discharge to the electric load, andsince the polymer electrolyte membrane is well humidified, this makes itpossible that the durability of the fuel cell 11 is ensuredsatisfactorily.

Besides, in accordance with the present embodiment, the drop in thetemperature, Tprox, of the selective oxidation unit 25 is suppressed inthe process of increasing the fuel gas dew point Tda, thereby preventingthe drop in the efficiency of lowering the carbon monoxide in theselective oxidation unit 25. This makes it possible to ensure thequality of fuel gas discharged from the selective oxidation unit 25.

Furthermore, in the present embodiment, the description has been made ofa manner in which, prior to cutting off the electric connection betweenthe fuel cell 11 and the load, the controller 20 provides, based on thefuel gas dew point or information relating thereto, control so that thewater is supplied to the anode of the fuel cell 11 to thereby increasethe fuel gas dew point, and thereafter the electric connection betweenthe fuel cell 11 with the load is cut off. However, such a manner shouldnot be deemed limitative. That is, it may be possible to employ a mannerin which, prior to cutting off the electric connection between the fuelcell 11 and the load, the controller 20 provides, based on the oxidizinggas dew point or information relating thereto, control so that the wateris supplied to the cathode of the fuel cell 11 to thereby increase theoxidizing gas dew point, and thereafter the electric connection betweenthe fuel cell 11 and the load is cut off. Alternatively, it may bepossible to employ a manner in which, prior to cutting off the electricconnection between the fuel cell 11 and the load, the controller 20performs control, based on at least one of either the fuel gas dew pointor information relating thereto and the oxidizing gas dew point orinformation relating thereto so that the water is supplied to at leastone of the anode and the cathode of the fuel cell 11 to thereby increaseat least one of the fuel gas dew point and the oxidizing gas dew point,and thereafter the electric connection between the fuel cell 11 and theload is cut off.

In addition, the remaining other configurations and operations are thesame as those of the fuel cell system 100 a, 100 b shown in FIGS. 1-5.

With the configuration of the present embodiment, it becomes possible toproperly supply water to the selective oxidation unit in the process ofshutting down the electric power generation operation of the fuel cellsystem. Specifically explaining with reference to FIG. 2, in accordancewith the configuration of the fuel cell system 100 a of the presentembodiment, it becomes possible to provide the supply, to the selectiveoxidation unit 25 of the fuel gas supplier 16, of water from the watertank 29 in the process of shutting down the electric power generationoperation of the fuel cell system 100 a by control of the operation ofthe water supply pump 28 by the controller 20. And, in this case, thecontroller 20 is able to properly control, based on either the dew pointof fuel gas or the information relating to the fuel gas dew point, theoperation of the water supply pump 28. Accordingly, with theconfiguration of the present embodiment, an adequate amount of waternecessary for properly increasing the fuel gas dew point is supplied tothe selective oxidation unit 25 of the fuel gas supplier 16 from thewater tank 29. This accomplishes a proper increase in the fuel gas dewpoint and, as a result, the fuel gas properly humidified by using onlyjust the necessary amount of water can be supplied to the polymerelectrolyte fuel cell by a simple configuration. As a result, it becomespossible to relatively increase the content of water of the polymerelectrolyte membrane, thereby making it possible to suppress thedegradation of the polymer electrolyte membrane. This makes it possibleto provide a fuel cell system having high durability.

Besides, with the configuration of the present embodiment, it ispossible to effectively utilize heat unnecessary in the selectiveoxidation unit. Specifically explaining with reference to FIG. 2, thereaction of combustion removal of carbon monoxide that develops in theselective oxidation unit 25 of the fuel gas supplier 16 is an exothermicreaction which generates heat. And, in accordance with the configurationof the present embodiment, water supplied to the selective oxidationunit 25 from the water tank 29 can be heated such that its temperatureapproaches closely to the fuel gas temperature in the selectiveoxidation unit 25 by the use of heat excessively generated by exothermicreaction in the selective oxidation unit 25 (i.e., unnecessary heat).This efficiently humidifies fuel gas in the selective oxidation unit 25.In the light of reducing the standby time until the operation of thefuel cell system is brought to a stop, such efficient humidification ofthe fuel gas proves to be an extremely effective means. Accordingly,with the configuration of the present embodiment, it becomes possible toeffectively use heat unnecessary in the selective oxidation unit 25.

Second Embodiment

In the first place, a configuration of a fuel cell system according to asecond embodiment of the present invention will be described withreference to FIG. 9.

FIG. 9 is a block diagram schematically illustrating a firstconfiguration of the fuel cell system according to the first embodimentof the present invention. Besides, in the present embodiment, a fuelcell and a cell provided in the fuel cell system have exactly the samecross sectional structures as those of the fuel cell and the cellprovided in the fuel cell system according to the first embodiment. Inthe following description, a description with regard to the crosssectional structures of the fuel cell and the cell provided in the fuelcell system is omitted accordingly.

As shown in FIG. 9, a fuel cell system 100 c according to the presentembodiment includes: a fuel cell 11 which generates electric power andheat when supplied with fuel gas and oxidizing gas; a fuel gas supplyline 58 for supplying the fuel gas to the fuel cell 11; an oxidizing gassupply line 59 for supplying oxidizing gas to the fuel cell 11; a fuelgas discharge line 60 to which surplus fuel gas from the fuel cell 11 isdischarged; an oxidizing gas discharge line 61 to which surplusoxidizing gas from the fuel cell 11 is discharged; a temperature controldevice 19 which regulates the temperature of the fuel cell 11; an outputcontroller 62 which draws electric power from the fuel cell 11; and acontroller 20 which suitably controls the operations of the fuel cellsystem 100 c.

Here, a description will be made with regard to a configuration of thefuel gas supply line 58.

As shown in FIG. 9, the fuel gas supply line 58 includes, as in the caseof the first embodiment, a fuel gas supplier 16 for producing fuel gasfrom raw material and oxidizing gas for oxidation reaction, and a dewpoint sensor 21 a for detecting the dew point of fuel gas supplied tothe fuel cell 11 from the fuel gas supplier 16. And, during the electricpower generation operation of the fuel cell system 100 c, after its dewpoint is detected by the dew point sensor 21 a, the fuel gas generatedin the fuel gas supplier 16 is supplied to the fuel cell 11.

The fuel gas supplier 16 includes, as in the case of the firstembodiment, a reformer 23, a shift converter 24 and a selectiveoxidation unit 25 and generates fuel gas rich in hydrogen from, forexample, town gas, propane gas, or other like gas. In addition, sulphurcomponents contained in the raw material are removed in a desulfurizer(not shown) and then is supplied to the reformer 23.

The reformer 23 employs, for example, a reforming method bysteam-reforming reaction and includes a vessel whose inside is filled upwith a catalyst of the nickel-based as a reform catalyst and a burner 49as a heat source. And, in the reformer 23, hydrogen-rich fuel gas isgenerated, by steam-reforming reaction, from supplied raw material andhigh-temperature water vapor. The fuel gas generated in the reformer 23is fed to the shift converter 24. In addition, the fuel gas generated inthe reformer 23 is rich in hydrogen but contains much carbon monoxidethat is harmful to the fuel cell 11.

The shift converter 24 includes, for example, a vessel filled with acatalyst of the copper-zinc-based or the like serving as a shiftcatalyst. In the shift converter 24, a predetermined shift reaction, inwhich carbon monoxide is oxidized to change to carbon dioxide, isexpedited whereby the carbon monoxide in the fuel gas generated in thereformer 23 is reduced. Then, after its carbon monoxide has been reducedin the shift converter 24, the fuel gas is supplied to the selectiveoxidation unit 25.

The oxidation unit 25 includes, for example, a vessel filled up with acatalyst of the platinum-alumina-based or the like serving as aselective oxidation catalyst. In the oxidation unit 25, a predeterminedselective oxidation reaction is made to advance by post-shift fuel gasfed from the shift converter 24 and oxidizing gas for selectiveoxidation (here air) drawn in from an oxidation-reaction oxidizing gassupply path 48, whereby the concentration of carbon monoxide present inthe fuel gas is reduced. In this way, the fuel gas whose carbon monoxidehas been reduced to a satisfactory level is supplied through the fuelgas supply path 12 to the fuel cell 11. In this way, the fuel gasgenerated in the fuel gas supplier 16 is humidified during thesteam-reforming reaction and then is supplied, with much water vapourcontained therein, to the fuel cell 11. The polymer electrolyte membrane1 of the fuel cell 11 is maintained in a predetermined wet state bymoisture contained in the fuel gas.

In addition, in the present embodiment, the fuel gas supplier 16 isprovided with a selective gas supply unit 52 for selectively supplyingoxidizing gas-containing cathode off-gas discharged from the fuel cell11 or oxidizing gas for oxidation reaction. More specifically, theselective gas supply unit 52 is made up of an oxidation-reactionoxidizing gas supply path 48 for supplying oxidation-reaction oxidizinggas to the selective oxidation unit 25, a first three-way valve 43disposed in the oxidation-reaction oxidizing gas supply path 48 andserving as an oxidizing gas supply path switch mechanism, and a cathodeoff-gas supply path 54 whose downstream end is connected to the firstthree-way valve 43. The first three-way valve 43 makes it possible toprovide selective switching to one of three states, namely a first statethat supplies oxidation-reaction oxidizing gas to the selectiveoxidation unit 25 of the fuel gas supplier 16, a second state thatsupplies cathode off-gas to the selective oxidation unit 25 of the fuelgas supplier 16, and a third state that supplies neitheroxidation-reaction oxidizing gas nor the supply of cathode off-gas tothe selective oxidation unit 25 of the fuel gas supplier 16.

In order to detect the dew point of fuel gas supplied to the fuel cell11 from the fuel gas supplier 16, the dew point sensor 21 a is disposedin the fuel gas supply path 12 through which the fuel gas generated inthe fuel gas supplier 16 to the fuel cell 11 is supplied. As well as inthe present embodiment, the dew point of fuel gas detected by the dewpoint sensor 21 a is regarded as serving as the fuel gas dew point Tdain the inside of the fuel cell 11. In addition, as in the case of thefirst embodiment, any type of dew point sensor may be employed as thedew point sensor 21 a as long as it has durability against fuel gasesand, in addition, temperature durability.

In addition, in the present embodiment, “Tda” indicates the temperaturewhen the total amount of moisture contained in the fuel gas is expressedin terms of the dew point temperature. Here, what is meant by “the totalcontent of moisture contained in the fuel gas” is the combined moisturecontent total that is the sum of the content of water vapor and thecontent of water in the fuel gas. For example, even if a part of themoisture contained in the fuel gas forms dew to thereby cause the fuelgas to contain water vapor and water, the temperature, found byexpressing, in terms of the dew point temperature, the combined moisturecontent total (i.e., the sum of the content of water vapor and thecontent of water in the fuel gas), serves as Tda on the basis of theabove-mentioned definition.

Next, a description will be made with regard to a configuration of theoxidizing gas supply line 59.

As shown in FIG. 9, the oxidizing gas supply line 59 is provided with anoxidizing gas supplier 17, a humidifier 18, and a dew point sensor 21 cfor detecting the dew point of oxidizing gas which is supplied to thefuel cell 11. Oxidizing gas from the oxidizing gas supplier 17 ishumidified in the humidifier 18 and, after its dew point is detected bythe dew point sensor 21 c, is supplied to the fuel cell 11.

The oxidizing gas supplier 17 is, for example, a sirocco fan, as in thecase of the first embodiment. The oxidizing gas supplier 17 draws inoxidizing gas (here, air) from the atmosphere and supplies the drawn-inoxidizing gas to the humidifier 18. Then, the humidifier 18 humidifiesthe oxidizing gas supplied from the oxidizing gas supplier 17 andthereafter delivers the humidified oxidizing gas to the fuel cell 11.

In order to detect the dew point of oxidizing gas supplied through thehumidifier 18 to the fuel cell 11 from the oxidizing gas supplier 17,the dew point sensor 21 c is disposed in the oxidizing gas supply path13 for supplying oxidizing gas to the fuel cell 11 from the humidifier18. As well as in the present embodiment, the dew point of oxidizing gasdetected by the dew point sensor 21 c is regarded as serving as theoxidizing gas dew point Tdc in the inside of the fuel cell 11. Inaddition, any type of dew point sensor may be employed as the dew pointsensor 21 c as long as it has durability against oxidizing gases and, inaddition, temperature durability.

In addition, in the present embodiment, “Tdc” indicates the temperaturewhen the total amount of moisture contained in the oxidizing gas isexpressed in terms of the dew point temperature. Here, what is meant by“the total content of moisture contained in the oxidizing gas” is thecombined moisture content total that is the sum of the content of watervapor and the content of water in the oxidizing gas. For example, evenif a part of the moisture contained in the oxidizing gas forms dew tothereby cause the oxidizing gas to contain both water vapor and water,the temperature, found by expressing, in terms of the dew pointtemperature, the moisture content total (the sum of the content of watervapor and the content of water in the oxidizing gas), serves as Tdc onthe basis of the above-mentioned definition.

Next, a description will be made with regard to a configuration of thefuel gas discharge line 60.

The fuel gas discharge line 60 is provided with an anode off-gasdischarge path 14 which connects together a fuel gas outlet port 50 ofthe fuel cell 11 and a fuel gas supply port of the burner 49. Of thefuel gas supplied to the fuel cell 11, unused fuel gas (surplus fuelgas) is discharged to the anode off-gas discharge path 14. Here, the gasthat contains fuel gas discharged to the anode off-gas discharge path 14from the fuel cell 11 will be called the “anode off-gas”. The anodeoff-gas discharge path 14 includes a heat exchanger (not shown) and acondenser (not shown) wherein fuel gas and water vapor contained in theanode off-gas are cooled in the heat exchanger and moisture is removedin the condenser. In this way, the fuel gas present in the anode off-gasis supplied to the burner 49 and is utilized there as a combustion fuel.

In addition, as shown in FIG. 9, connected to the anode off-gasdischarge path 14 is the downstream end of a bypass path 47 throughwhich fuel gas is fed, bypassing the fuel cell 11, to the burner 49 fromthe fuel gas supply path 12. Disposed upstream of where the anodeoff-gas discharge path 14 and the bypass path 47 are connected is anon-off valve 46 which opens and closes the anode off-gas discharge path14. Besides, a third three-way valve 45 is disposed at where theupstream end of the bypass path 47 and the fuel gas supply path 12 areconnected together. The third three-way valve 45 makes it possible toprovide selective switching to one of states, a namely a first statethat allows the supply of fuel gas generated in the fuel gas supplier 16to the fuel cell 11 and another state that allows the supply of fuel gasto the burner 49 in which supply the fuel cell 11 is bypassed.

Next, a description will be made with regard to a configuration of theoxidizing gas discharge line 61.

The oxidizing gas discharge line 61 is provided with a cathode off-gasdischarge path 15 the upstream end of which is connected to an oxidizinggas outlet port 51 of the fuel cell 11. Surplus oxidizing gas remainingunused and water generated in the fuel cell 11 are discharged to thecathode off-gas discharge path 15. Here, the gas with a content ofoxidizing gas discharged to the cathode off-gas discharge path 15 iscalled the “cathode off-gas”. The cathode off-gas containshigh-temperature oxygen, nitrogen and water vapor. The cathode off-gasdischarge path 15 is provided with a second three-way valve 44 servingas a cathode off-gas discharge path switch mechanism and, by the secondthree-way valve 44, the cathode off-gas discharge path 15 is branchedoff into a discharge path and a supply path to the selective oxidationunit 25 of the fuel gas supplier 16. As a supply path to the selectiveoxidation unit 25 of the fuel gas supplier 16, a cathode off-gas supplypath 54 is provided whose upstream end is connected to the secondthree-way valve 44. The downstream end of the cathode off-gas supplypath 54 is connected to the first three-way valve 43 which is disposedin the oxidation-reaction oxidizing gas supply path 48 through which todeliver oxidation-reaction oxidizing gas to the selective oxidation unit25 of the fuel gas supplier 16 and which serves as an oxidizing gassupply path switch mechanism. In this way, the fuel cell system 100 caccording to the present embodiment is configured so that a part or allof the cathode off-gas discharged from the fuel cell 11 can be suppliedthrough the cathode off-gas supply path 54 to the selective oxidationunit 25 of the fuel gas supplier 16. In addition, the downstream end ofthe cathode off-gas discharge path 15 is opened to the atmosphere.

Next, a description will be made with regard to a configuration of thetemperature control device 19.

The temperature control device 19 includes, as in the case of the firstembodiment, a circulation path 53 and a circulation pump (not shown) forcirculating cooling medium between the temperature control device 19 andthe fuel cell 11, and a heat radiator such as a cooling fin, a heatexchanger et cetera for causing cooling medium circulating in thecirculation path 53 to release heat therefrom. In the temperaturecontrol device 19, cooling medium circulating in the circulation path 53is supplied to the fuel cell 11, is discharged from the fuel cell 11after having been heated to higher temperature by heat generatedassociated with the generation of electric power, is cooled to lowertemperature in the heat radiator, and thereafter is again supplied tothe fuel cell 11. In addition, heat energy recovered from the fuel cell11 by cooling medium is used for supplying hot water or other likeapplication.

Besides, the temperature control device 19 is configured such that thetemperature of the fuel cell 11 is maintained at a constant level bymaking a change in at least either one of the flow rate and the amountof heat radiation of the cooling medium. For the temperature control ofthe fuel cell 11, the temperature control device 19 is provided with atemperature sensor 22 for detecting the temperature of cooling medium.In the present embodiment, the temperature sensor 22 is disposed so asto be able to detect the temperature of cooling medium discharged to thetemperature control device 19 from the fuel cell 11. As a method fordetecting the temperature of the fuel cell 11, it is conceivable toemploy a method which measures the temperature of cooling mediumsupplied to the fuel cell 11, a method which directly measures thetemperature of separators 7 a and 7 c which compose the fuel cell 11 bymeans of a thermocouple, a method which measures the temperature ofcooling medium discharged from the fuel cell 7, or other like method.

Also in the present embodiment, the temperature of cooling mediumdetected by the temperature sensor 22 is regarded as serving as the“temperature, Tcell, of the fuel cell 11”. In addition, the temperature,Tcell, of the fuel cell 11 is the highest of the temperatures of thefuel cell 11. On the other hand, the portion that has the highest of thetemperatures of the fuel cell 11 is supposed to be an outlet portportion from which cooling medium supplied from the inlet port exits.Therefore, in the present embodiment, it is configured such that thetemperature of cooling medium discharged to the temperature controldevice 19 from the fuel cell 11 is detected using the temperature sensor22.

Next, a description will be made with regard to a configuration of theoutput controller 62.

The input and the output terminals of the output controller 62 areconnected to the output terminal of the fuel cell 11 and to the electricload, respectively. Here, the output controller 62 is provided with aninverter so that a direct current generated in the fuel cell 11 isconverted into an alternating current which is then output to the load.Besides, the output controller 62 controls an electric current (output)drawn out from the fuel cell 11 to thereby control the electric powergeneration amount of the fuel cell 11. The output controller 62establishes the electric connection/disconnection between the fuel cell11 and the electric load.

Last of all, a description will be made with regard to a configurationof the controller 20.

As in the case of the first embodiment, the controller 20 according tothe present embodiment includes, for example, an MPU and a memory and,based on data such as programs, parameters, or the like prestored in thememory, suitably controls the operation of each of elements constitutingthe fuel cell system 100. In the present embodiment, the controller 20suitably controls the operations of at least the fuel cell 11, the fuelgas supplier 16, the oxidizing gas supplier 17 and the temperaturecontrol device 19 and, in addition, suitably controls the operations ofthe first three-way valve 43, the second three-way valve 44, and theoutput controller 62.

Next, referring to FIG. 10, a modification of the fuel cell systemaccording to the second embodiment of the present invention will bedescribed below.

FIG. 10 is a block diagram schematically illustrating a secondconfiguration of the fuel cell system according to the second embodimentof the present invention.

As shown in FIG. 10, there is shown a fuel cell system 100 d accordingto the second embodiment of the present invention which system hasbasically the same configuration as the configuration of the fuel cellsystem 100 a shown in FIG. 9. That is, the fuel cell system 100 dincludes a fuel gas supplier 16, an oxidizing gas supplier 17, ahumidifier 18, a dew point sensor 21 a, a dew point sensor 21 c, a fuelcell 11, a temperature control device 19, an output controller 62 and acontroller 20. And, the fuel cell system 100 d, too, is configured suchthat fuel gas is supplied to the fuel cell 11 from the fuel gas supplier16 through the dew point sensor 21 a while oxidizing gas is supplied tothe fuel cell 11 from the oxidizing gas supplier 17 through thehumidifier 18 and then through the dew point sensor 21 c. Besides, thefuel cell system 100 d, too, is configured such that the temperature ofthe fuel cell 11 is controlled by the temperature control device 19while the operations of the fuel cell system 100 d is suitablycontrolled by the controller 20.

On the other hand, contrary to the configuration of the fuel cell system100 c that the oxidation-reaction oxidizing gas and so on are suppliedthrough the oxidation-reaction oxidizing gas supply path 48 to theselective oxidation unit 25 of the fuel gas supplier 16, the fuel cellsystem 100 d of the present embodiment (FIG. 10) is configured such thatthe oxidation-reaction oxidizing gas and so on are supplied through theoxidation-reaction oxidizing gas supply path 48 to the connecting pipingof the shift converter 24 and the selective oxidation unit 25, as shownin FIG. 10. In addition, the rest is the same as the fuel cell system100 c shown in FIG. 9.

Next, a description will be made in detail with respect to acharacteristic operation of the fuel cell system according to the secondembodiment of the present invention. In addition, the hereinafterdescribed characteristic operation of the fuel cell system 100 c, 100 dis carried out by the execution of a predetermined program prestored inthe memory of the controller 20 in the MPU.

The fuel cell system 100 c, 100 d according to the present embodimenthas four different operational modes, i.e., a “shutdown state (standbystate)” in which all the constituent elements concerned with thegeneration of electric power are shut down; an “electric powergeneration operation” in which electric power is generated; a “start-upoperation” in which the fuel cell system 100 c, 100 d is activated, in asmooth manner, to the “electric power generation operation” from the“shutdown state”; and a “shutdown operation” in which the fuel cellsystem 100 c, 100 d is shut down, in a smooth manner, to the “shutdownstate” from the “electric power generation operation”.

In the present embodiment, the constituent elements other than thecontroller 20 are shut down in the “shutdown state”. In the presentinvention, the time that the “start-up operation” starts is the outputtime of a “start-up signal” in the fuel cell system 100 c, 100 d. Thetime that the “shutdown operation” starts is the output time of a“shutdown signal” in the fuel cell system 100 c, 100 d. The time thatthe “generation operation” starts is the time that the fuel cell 11starts generating electric power. Accordingly, the “start-up operationtime” means the period of time from when the “start-up signal” is outputin the fuel cell system 100 c, 100 d to when the fuel cell 11 startsgenerating electric power. The “electric power generation operationtime” means the period of time from when the fuel cell 11 startsgenerating electric power to when the “shutdown signal” is output in thefuel cell system 100 c, 100 d. The “shutdown operation time” means theperiod of time from when the “shutdown signal” is output in the fuelcell system 100 c, 100 d to when all of the constituent elementsconcerned with the generation of electric power are shut down. Besides,the time that the fuel cell 11 is electrically connected to the electricload to enter the closed circuit state is called the “electric powergenerating time” while on the other hand the time that the fuel cell 11is electrically cut off from the electric load to enter the open circuitstate is the “non electric power generating time”.

In the present embodiment, during the electric power generationoperation, the fuel cell 11 generates electric power in an adequate lowhumidification state in the fuel cell system 100 c, 100 d. This includesa case in which al least a part of the inside of the fuel cell 11 is ina low humidification state. In addition, here, the “low humidificationstate” is, for example, an operational condition in which: (thecooling-medium inlet port portion's temperature in the fuel cell11≦Tda<(the cooling-medium outlet port portion's temperature in the fuelcell 11, i.e., Tcell) and, in addition, (the cooling-medium inlet portportion's temperature in the fuel cell 11≦Tdc<(the cooling-medium outletport portion's temperature in the fuel cell 11, i.e., Tcell).

However, the low humidification state may be any operational conditionthat meets at least both the interrelations: Tcell>Tda and Tcell>Tdc.Accordingly, the low humidification state may include, for example, anoperational condition in which: Tda<(the cooling-medium inlet portportion's temperature in the fuel cell 11) and, in addition, Tdc<(thecooling-medium inlet port portion's temperature in the fuel cell 11). Inthis case, almost the entire area of the inside of the fuel cell 11 isin the low humidification condition. Besides, the low humidificationcondition may include, for example, an operational condition in which:(the cooling-medium inlet port portion's temperature in the fuel cell11)≦Tda<(the cooling-medium outlet port portion's temperature in thefuel cell 11, i.e., Tcell) and, in addition, Tdc<(the cooling-mediuminlet port portion's temperature in the fuel cell 11). Alternatively,the low humidification state may include, for example, an operationalcondition in which: Tda<(the cooling-medium inlet port portion'stemperature in the fuel cell 11≦Tdc<(the cooling-medium outlet portportion's temperature in the fuel cell 11, i.e., Tcell).

The operation of the fuel cell system 100 c, 100 d according to thepresent embodiment is the same as the operation of the conventional fuelcell system with the exception that, when the shutdown operation of thefuel cell system 100 c, 100 d starts, the cathode off-gas is supplied tothe selective oxidation unit 25 of the fuel gas supplier 16 prior tocutting off the electric connection between the fuel cell 11 which isgenerating electric power in the low humidification state, and theelectric load (that is, before the fuel cell 11 enters the open circuitstate). Therefore, hereinafter, only characteristic operations of thefuel cell system 100 c, 100 d according to the present embodiment willselectively described in detail.

FIG. 11 is a flow chart schematically representing a characteristicoperation of the fuel cell system according to the second embodiment ofthe present invention. It should be noted that FIG. 11 selectivelyrepresents only steps necessary for describing the present invention,and the diagrammatic representation of the other remaining steps isomitted.

Additionally, FIG. 12 is a time chart schematically representing how thefuel cell temperature Tcell, the fuel gas dew point Tda, the oxidizinggas dew point Tdc, the selective oxidation unit temperature Tprox andthe fuel cell output voltage Vfc each vary in the characteristicoperation of the fuel cell system according to the second embodiment ofthe present invention. It should be noted that FIG. 12 selectivelyrepresents only operations necessary for describing the presentinvention, and the diagrammatic representation of the other remainingoperations is omitted.

As shown in FIGS. 11 and 12, during the electric power generationoperation, the fuel cell 11 of the fuel cell system 100 c, 100 dgenerates electric power in the low humidification state (State 1). InState 1 shown in FIG. 12, when the shutdown operation starts in the fuelcell system 100 c, 100 d, the controller 20 first causes the secondthree-way valve 44 to operate so that the cathode off-gas is supplied tothe selective oxidation unit 25 of the fuel gas supplier 16, whilemaintaining the electric connection between the fuel cell 11 and theeclectic load, in other words, while the generation of electric power ismaintained (Step S1; Operation 1 of FIG. 12). As a result of this, theselective oxidation unit 25 of the fuel gas supplier 16 is supplied withcathode off-gas containing much moisture, whereby the fuel gas dew pointTda increases (State 2). In addition, the cathode off-gas containswater, in the form of water vapor, generated by the reaction of fuel gasand oxidizing gas in the fuel cell 11.

In State 2 shown in FIG. 12, although both the oxidizing gas dew pointTdc and the temperature, Tcell, of the fuel cell 11 remain almostunchanged because the electric power generation of the fuel cell 11 ismaintained, the fuel gas dew point Tda increases with time while thetemperature, Tprox, of the selective oxidation unit 25 decreases withtime because the cathode off-gas containing much moisture is supplied tothe selective oxidation unit 25 of the fuel gas supplier 16.

In addition, the rate of flow of the cathode off-gas supplied to theselective oxidation unit 25 of the fuel gas supplier 16 is suitablyregulated. For example, it is possible to control the rate of flow ofthe cathode off-gas supplied to the selective oxidation unit 25 of thefuel gas supplier 16 by regulating the flow ratio of cathode off-gasbetween in the supply path and in the discharge path in the secondthree-way valve 44. In this case, preferably a force feed means, such asa pump or the like, is provided so that cathode off-gas is force-fed tothe cathode off-gas supply path 54. Alternatively, the rate of flow ofthe cathode off-gas supplied to the selective oxidation unit 25 of thefuel gas supplier 16 can be controlled by operating the second three-wayvalve 44 so that all cathode off-gas flows into the cathode off-gassupply path 54 and, in addition, by regulating the rate of flow of theoxidizing gas supplied to the fuel cell 11 by the oxidizing gas supplier17.

Next, the controller 20 makes a decision of whether or not thetemperature, Tcell, of the fuel cell 11 conforms to the fuel gas dewpoint Tda, while causing the fuel cell 11 to continue generatingelectric power (Step S2). Here, based on the values detected by the dewpoint sensor 21 a and by the temperature sensor 22, the controller 20makes a decision on the magnitude relation between the fuel gas dewpoint Tda and the temperature, Tcell, of the fuel cell 11.

The fuel gas dew point Tda increases, and when the fuel gas dew pointTda and the temperature, Tcell, of the fuel cell 11 finally conform toeach other (“YES” in Step S2), the controller 20 causes the secondthree-way valve 44 to operate so that the supply of cathode off-gas tothe selective oxidation unit 25 of the fuel gas supplier 16 is broughtto a stop, while the electric connection between the fuel cell 11 andthe electric load is maintained (Step S3; Operation 2 of FIG. 12). Thisstops the increase in the fuel gas dew point Tda.

Here when the temperature, Tcell, of the fuel cell 11 and the fuel gasdew point Tda conform to one another, the controller 20 stops the supplyof cathode off-gas to the selective oxidation unit 25 of the fuel gassupplier 16. However, the advantageous effects of the present inventioncan be obtained if the condition for stopping the supply of cathodeoff-gas to the selective oxidation unit 25 of the fuel gas supplier 16is: Tda (the fuel gas dew point)>Tcell (the temperature of the fuel cell11).

Then the controller 20 controls the second three-way valve 44 to operateso that the supply of cathode off-gas to the selective oxidation unit 25is stopped, subsequently starts a time measurement, and makes a decisionof whether or not the measured time Tm reaches a preset predeterminedtime Tpd (Step S4). Here if the decision made in this Step S4 indicatesthat the measured time Tm does not reach the preset predetermined timeTpd (“NO” in Step S4), the controller 20 provides control that the stateof the fuel cell system 100 is maintained until the time the measuredtime Tm reaches the preset predetermined time Tpd, while the electricpower generation of the fuel cell 11 is continued (Step 3). In Step S3shown in FIG. 12, the electric power generation of the fuel cell 11 iscontinued, and the polymer electrolyte membrane 1 of the fuel cell 11 iswell humidified, by moisture contained in the fuel and oxidizing gases,up to a state capable of preventing the polymer electrolyte membranefrom degradation.

Eventually, if the measured time Tm reaches the preset predeterminedtime Tpd (“YES in Step S4), the controller 20 provides control that cutsoff the electric connection between the fuel cell 11 and the electricload (Step S5; Operation 3 of FIG. 12). As a result of this, thegeneration of electric power of the fuel cell 11 ceases, and the fuelcell 11 makes a change in its state to the open circuit state from theclosed circuit state (State 4).

In State 4 shown in FIG. 12, although the output voltage, Vfc, of thefuel cell 11 is increased by the state change to the open circuit state,the wet state of the polymer electrolyte membrane 1 of the fuel cell 11is higher than the wet state of the polymer electrolyte membrane 1during the electric power generation operation in the low humidificationstate. The reason for this is explained as follows. That is, theinterrelation: Tcell (the fuel cell temperature)≦Tda (the fuel gas dewpoint) holds because the fuel gas dew point is increased by moisturecontained in the cathode off-gas and, therefore, the polymer electrolytemembrane 1 is well humidified by moisture contained in the fuel gas. Inthis way, in the fuel cell 11, during the non electric power generation,the polymer electrolyte membrane 1 is well humidified, thereby reducingthe swelling/contraction of the polymer electrolyte membrane 1. As aresult of this, it becomes possible to prevent the polymer electrolytemembrane 1 from damage (such as break or the like) associated with theincrease in the stop frequency of the electric power generation of thepolymer electrolyte fuel cell 1, whereby the degradation thereof issuppressed.

Thereafter, the controller 20 controls and causes the fuel gas supplier16 and the oxidizing gas supplier 17 to stop operating. Here, it isdesirable that the selective oxidation unit 25 of the fuel gas supplier16 is filled up with raw material before the operation of the fuel gassupplier 16 is stopped and when the fuel cell 11 is in the open circuitstate. More specifically, after cutting off the electric connectionbetween the fuel cell 11 and the electric load in Step S5, thecontroller 20 controls the first three-way valve 43 to operate so thatthe supply of oxidizing gas and cathode off-gas is stopped, whilemaintaining the supply of raw material. Furthermore, the third three-wayvalve 45 is switched to the bypass path 47 and the on-off valve 46 isclosed, thereby establishing a state in which the fuel gas present inthe inside of the selective oxidation unit 25 is supplied to the burner49 without passage through the fuel cell 11. By the filling up of theselective oxidation unit 25 of the fuel gas supplier 16 with rawmaterial in the way as described above, the selective oxidation unit 25becomes dried, thereby making it possible to prevent water fromcondensing in the selective oxidation unit 25 during the shutdown of thefuel cell system 100 c, 100 d. This makes it possible to suppress thedegradation of the selective oxidation catalyst mounted in the selectiveoxidation unit 25 of the fuel gas supplier 16. Furthermore, since thereis no increase in the starting energy of the selective oxidation unit 25of the fuel gas supplier 16, this contributes to increasing theefficiency of the fuel cell system 100 c, 100 d to higher level.

Finally, the controller 20 controls and causes all the constituentelements concerned with the electric power generation of the fuel cellsystem 100 c, 100 d to stop operating and then places the fuel cellsystem 100 c, 100 d in the shutdown state.

As described above, in the present embodiment, the moisture-containingcathode off-gas is supplied to the selective oxidation unit 25 of thefuel gas supplier 16 when stopping the electric power generation of thefuel cell 11 from the state in which it is generating electric power inthe low humidification state, whereby Tda, the dew point of fuel gassupplied to the fuel cell 11, increases. The electric power generationof the fuel cell 11 is continued in this state and is stoped after thefuel gas dew point Tda increases to conform to the temperature, Tcell,of the fuel cell 11. As a result of this, the interrelation: Tcell (thetemperature of the fuel cell 11)≦Tda (the fuel gas dew point) holds inthe fuel cell 11 during the non electric power generation. Now,therefore, during the non electric power generation of the fuel cell 11,the polymer electrolyte membrane 1 is well humidified, whereby thedegradation of the polymer electrolyte membrane 1 is suppressed tothereby enhance the durability of the fuel cell 11.

In addition, in the present embodiment, the description has been madewith respect to an embodiment in which the dew point sensor 21 a and thedew point sensor 21 c are used to detect the fuel gas dew point Tda andthe oxidizing gas dew point Tdc. However, such an embodiment should notbe deemed limitative. For example, as in the first embodiment, from thefact that the fuel gas dew point Tda is dependent on the performance ofthe fuel gas supplier 16 while the oxidizing gas dew point Tdc isdependent on the performance of the humidifier 18, it may be possible toemploy a manner in which the fuel gas dew point Tda is a dew pointcalculated based on the operational condition of the fuel gas supplier16 (for example, a parameter such as the rate of flow of raw material,the amount of reforming water, the reforming temperature et cetera)while the oxidizing gas dew point Tdc is either a dew point calculatedbased on the operational condition of the humidifier 18 or thetemperature of the humidifier 18. In this case, based on the valuedetected by the temperature sensor 22 and on the elapsed time since thestart of the supply of cathode off-gas to the selective oxidation unit25, the controller 20 makes a decision on the magnitude relation betweenthe fuel gas dew point Tda and the temperature, Tcell, of the fuel cell11.

Besides, in the present embodiment, the description has been made of anembodiment in which the electric connection between the fuel cell 11 andthe electric load is cut off after the measured time Tm reaches thepreset predetermined time Tpd. However, such a manner should not bedeemed limitative. For example, it may be possible to employ a manner inwhich the electric connection between the fuel cell 11 and the electricload is cut off upon the conformity of the temperature, Tcell, of thefuel cell 11 with the fuel gas dew point Tda. Even in such a manner, thesame advantageous effects as accomplished by the present embodiment canbe accomplished.

Besides, in the present embodiment, the description has been made withrespect to an embodiment in which the electric power generation of thefuel cell 11 is brought to a stop after Tda (the dew point of fuel gassupplied to the fuel cell 11) increases to conform to Tcell (thetemperature of the fuel cell 11). However, such a manner should not bedeemed limitative. For example, it may be possible to employ a manner inwhich the electric power generation of the fuel cell 11 is brought to astop after Tda (the dew point of fuel gas which is supplied to the fuelcell 11) increases to exceed the temperature, Tcell, of the fuel cell11. As a result of this, the interrelation: the temperature, Tcell, ofthe fuel cell 11)<(the fuel gas dew point Tda) holds in the fuel cell 11during the non electric power generation of the fuel cell 11. As aresult of this, during the non electric power generation of the fuelcell 11, the polymer electrolyte membrane 1 is well humidified, wherebythe degradation of the polymer electrolyte membrane 1 is ensuredlysuppressed.

Next, referring to FIG. 13, a second modification of the fuel cellsystem according to the second embodiment of the present invention willbe described.

FIG. 13 is a block diagram schematically illustrating a thirdconfiguration of the fuel cell system according to the second embodimentof the present invention.

The third configuration of the fuel cell system according to the secondembodiment of the present invention is the same as that of the fuel cellsystem 100 c (shown in FIG. 9), with the exception that neither thefirst three-way valve 43 nor the second three-way valve 44 (shown inFIG. 9) is provided and, in addition, the oxidizing-reaction oxidizinggas is supplied to the selective oxidation unit 25 of the fuel gassupplier 16 from the oxidizing gas supplier 17. Accordingly, here, adescription will be made with respect to the difference between thethird configuration of the fuel cell system and the configuration of thefuel cell system 100 c shown in FIG. 9, and the description with respectto portions in common is omitted.

Referring to FIG. 13, there is shown a fuel cell system 100 e in whichthe oxidizing-reaction oxidizing gas is supplied through theoxidizing-reaction oxidizing gas supply path 48 to the selectiveoxidation unit 25 of the fuel gas supplier 16 from the oxidizing gassupplier 17. The oxidizing-reaction oxidizing gas supply path 48 isprovided with a variable orifice 55 as a flow rate regulator forregulating the supply amount of or the flow rate of oxidizing-reactionoxidizing gas supplied to the selective oxidation unit 25. On the otherhand, in the fuel cell system 100 e, the cathode off-gas discharged fromthe oxidizing gas outlet port 51 of the fuel cell 11 is supplied throughthe cathode off-gas supply path 54 to the selective oxidation unit 25 ofthe fuel gas supplier 16. The cathode off-gas supply path 54 is acathode off-gas passage which is branched off from the cathode off-gasdischarge path 15 connected to the oxidizing gas outlet port 51 of thefuel cell 11. The cathode off-gas supply path 54 is provided with a pump56 for regulating the supply amount of or the flow rate of cathodeoff-gas supplied to the selective oxidation unit 25, and for forciblyfeeding cathode off-gas. The variable orifice 55 and the pump 56together form a selective gas supply unit 57.

And, during the electric power generation of the fuel cell 11, thevariable orifice 55 is opened to a predetermined opening degree, and theoxidizing-reaction oxidizing gas is supplied to the selective oxidationunit 25 of the fuel gas supplier 16 from the oxidizing gas supplier 17.At this time, no cathode off-gas will flow into the cathode off-gassupply path 54 because the pump 56 is shut down and the fuel gas presentin the selective oxidation unit 25 will not flow into or flow back tothe cathode off-gas supply path 54.

Besides, when supplying the cathode off-gas to the selective oxidationunit 25 of the fuel gas supplier 16 during the shutdown operation of thefuel cell system 100 e, the controller 20 causes the pump 56 to operateso that cathode off-gas is force-fed through the cathode off-gas supplypath 54 to the selective oxidation unit 25, and places the variableorifice 55 in the closed state. This results in the supply of cathodeoff-gas rich in moisture to the selective oxidation unit 25 of the fuelgas supplier 16, thereby increasing the dew point of fuel gas suppliedto the fuel cell 11 from the selective oxidation unit 25. In addition,the amount of the supply of cathode off-gas to the selective oxidationunit 25 is regulated by the force-feed capability of the pump 56.

Besides, when stopping the supply of cathode off-gas to the selectiveoxidation unit 25 of the fuel gas supplier 16 during the shutdownoperation of the fuel cell system 100 e, the controller 20 causes thepump 56 to shut down to thereby stop the supply of cathode off-gas tothe selective oxidation unit 25. As described above, during the shutdownoperation and the shutdown of the fuel cell system 100 e, the polymerelectrolyte membrane 1 of the fuel cell 11 is well humidified bymoisture contained in the fuel gas and is maintained in an adequatehumidification state. Therefore, the degradation of the polymerelectrolyte membrane 1 mounted in the fuel cell 11 is suppressed tothereby contribute to the extension of the life of the fuel cell 11.

Besides, here, although the description has been made with respect to amanner in which the variable orifice 55 is provided as a flow rateregulator, such manner should not be deemed limitative. For example, itmay be possible to use, as a flow rate regulator, a fixed orifice and anon-off valve. Even in this manner, the same advantageous effects asaccomplished by the present embodiment can be accomplished.

In addition, although, in the present embodiment, the selective gassupply unit 52, 57 completely selectively supplies oxidizing-reactionoxidizing gas or cathode off-gas to the selective oxidation unit 25 ofthe fuel gas supplier 16, it may be possible to supply the both gases ata predetermined mixing ratio. In the present embodiment, the gas issupplied, in this way, to the selective oxidation unit 25 of the fuelgas supplier 16 by the selective gas supply unit 52, 57 includes notonly a case in which oxidizing-reaction oxidizing gas or cathode off-gasis completely selectively supplied but also a case in which the bothgases are supplied at a predetermined mixing ratio.

Third Embodiment

In the first and the second embodiments of the present invention, thedescription has been made with respect to the manner in which the fuelcell system is equipped with a fuel gas supplier. On the other hand, inthe case of employing a fuel cell system as a drive electric powersource for an electric vehicle, a hydrogen cylinder is usually used as asubstitute for a fuel gas supplier. Therefore, in the third embodimentof the present invention, a description will be made with respect to anembodiment in which the fuel cell system includes, in place of a fuelgas supplier, a hydrogen cylinder.

First, with reference to FIG. 14, a description will be made withrespect to a first configuration of a fuel cell system according to thethird embodiment of the present invention and with respect to itsoperation.

FIG. 14 is a block diagram schematically illustrating the firstconfiguration of the fuel cell system according to the third embodimentof the present invention. It should be noted that FIG. 14 selectivelyshows only constituent components necessary for describing the presentinvention, and the diagrammatic representation of the remaining otherconstituent components is omitted. Besides, also in the presentembodiment, the cross sectional structure of a fuel cell provided in thefuel cell system is exactly the same as the cross sectional structure ofa fuel cell provided in the fuel cell system according to the firstembodiment. In the following description, a description about the crosssectional structure of the fuel cell provided in the fuel cell system isomitted accordingly.

Besides, the fuel cell system according to the present embodiment hasthe same configuration as the fuel cell system 100 a shown in FIG. 2,with the exception of the following differences: the fuel gas supplier16 and the on-off valves 26 and 27, and the humidifier 18 (see FIG. 2)are not provided; hydrogen (fuel gas) is supplied to the fuel cell froma hydrogen cylinder; and the supply of water is provided to hydrogenfrom the hydrogen cylinder. Accordingly, in the present embodiment, adescription will be made with respect to the difference between thefirst configuration of the fuel cell system and the configuration of thefuel cell system 100 a shown in FIG. 2, and a description with respectto portions in common therebetween is omitted.

As shown in FIG. 14, in a fuel cell system 100 f according to thepresent embodiment, the hydrogen (fuel gas) is supplied through theon-off valve 41 (a hydrogen supply mechanism) to the dew point sensor 21a from a hydrogen cylinder 30 (a fuel gas supply unit). Here, theoperation of the on-off valve 41 is controlled by the controller 20,whereby the supply amount of or the flow rate of hydrogen supplied tothe dew point sensor 21 a from the hydrogen cylinder 30 is controlled.Besides, in the fuel cell system 100 f, the oxidizing gas is supplied tothe dew point sensor 21 c from the oxidizing gas supplier 17. Meanwhile,in the fuel cell system 100 f, the water is supplied through the watersupply pump 28 to the dew point sensor 21 a from the water tank 29, asshown in FIG. 14. More specifically, water supplied through the watersupply pump 28 from the water tank 29 is introduced to hydrogen suppliedthrough the on-off valve 41 from the hydrogen cylinder 30. That is, thefuel cell system 100 f according to the present embodiment is configuredsuch that hydrogen from the hydrogen cylinder 30 and water from thewater tank 29 can be supplied to the dew point sensor 21 a at the sametime. Here, the operation of the water supply pump 28 is controlled bythe controller 20, whereby the supply amount of or the flow rate ofwater supplied to the dew point sensor 21 a from the water tank 29 iscontrolled.

And, during the electric power generation of the fuel cell 11, theon-off valve 41 is opened to a predetermined opening degree and thehydrogen (fuel gas) is supplied through the dew point sensor 21 a andthen through the fuel gas supply path 12 to the anode of the fuel cell11 from the hydrogen cylinder 30. At this time, there is no inflow ofhydrogen to the water tank 29 from the hydrogen cylinder 30 because thewater supply pump 28 is shut down. Besides, during the electric powergeneration of the fuel cell 11, the supply, to the cathode of the fuelcell 11, of oxidizing gas is provided through the dew point sensor 21 cfrom the oxidizing gas supplier 17.

Besides, when supplying water to hydrogen supplied from the hydrogencylinder 30 during the shutdown operation of the fuel cell system 100 f,the controller 20 causes the water supply pump 28 to operate so thatwater is force-fed to the dew point sensor 21 a from the water tank 29.As a result of this, water from the water tank 29 is fed to hydrogenfrom the hydrogen cylinder 30, thereby increasing the dew point of fuelgas supplied to the fuel cell 11 from the dew point sensor 21 a. Inaddition, the supply amount of water to hydrogen from the hydrogencylinder 30 is regulated by the force-feed capability of the watersupply pump 28. Besides, at this time, the on-off valve 41 is opened toa predetermined opening degree, but there is no inflow of water to thehydrogen cylinder 30 because hydrogen is supplied towards the dew pointsensor 21 a from the hydrogen cylinder 30.

Besides, when stopping the supply of water to hydrogen from the hydrogencylinder 30 during the shutdown operation of the fuel cell system 100 f,the controller 20 causes the operation of the water supply pump 28 tostop to thereby bring the supply of water to hydrogen from the hydrogencylinder 30 to a stop. As described above, during the shutdown operationand the shutdown of the fuel cell system 100 f, the polymer electrolytemembrane 1 of the fuel cell 11 is well humidified by moisture containedin the fuel gas and will be maintained in an adequate humidificationstate. Therefore, by the present embodiment, the degradation of thepolymer electrolyte membrane 1 mounted in the fuel cell 11 is suppressedto thereby contribute to the extension of the life of the fuel cell 11.

Besides, in the present embodiment, although the description has beenmade with respect to an embodiment in which hydrogen from the hydrogencylinder 30 is supplied with liquid water, such a manner should not bedeemed limitative. For example, it may be possible to employ a manner inwhich the fuel cell system 100 f includes a spray for atomizing liquidwater, and water atomized by the spray is supplied to hydrogen from thehydrogen cylinder 30. Even in this manner, the same advantageous effectsas accomplished by the present embodiment can be accomplished.

Furthermore, the rest is the same as the first and the secondembodiments.

Next, referring to FIG. 15, a second configuration of the fuel cellsystem according to the third embodiment of the present invention willbe described.

FIG. 15 is a block diagram schematically illustrating the secondconfiguration of the fuel cell system according to the third embodimentof the present invention.

As shown in FIG. 15, a fuel cell system 100 g according to the thirdembodiment of the present invention has a configuration basicallyidentical with the configuration of the fuel cell system 100 f (FIG.14). That is, the fuel cell system 100 g includes a hydrogen cylinder30, an on-off valve 41, an oxidizing gas supplier 17, a dew point sensor21 a, a dew point sensor 21 c, a fuel cell 11, a temperature controldevice 19, a temperature sensor 22 and a controller 20. And, the fuelcell system 100 g is configured such that the supply, to the fuel cell11, of hydrogen from the hydrogen cylinder 30 is provided through theon-off valve 41 and then through the dew point sensor 21 a and thesupply, to the fuel cell 11, of oxidizing gas from the oxidizing gassupplier 17 is provided through the dew point sensor 21 c. Besides, thefuel cell system 100 g is configured such that the temperature of thefuel cell 11 is controlled by the temperature control device 19 and theoperations of the fuel cell system 100 g are suitably controlled by thecontroller 20.

On the other hand, contrary to the fuel cell system 100 f which isconfigured such that the water is supplied from the water tank 29through the water supply pump 28 to hydrogen from the hydrogen cylinder30, the fuel cell system 100 g according to the present embodiment isconfigured such that the water is supplied from the water tank 29through the water supply pump 28 to oxidizing gas from the oxidizing gassupplier 17, as shown in FIG. 15. In addition, the rest is the same asthe fuel cell system 100 f shown in FIG. 14.

Even when configured such that the water is supplied from the water tank29 through the water supply pump 28 to oxidizing gas from the oxidizinggas supplier 17, the polymer electrolyte membrane 1 of the fuel cell 11is well humidified by moisture contained in the oxidizing gas during theshutdown operation and the shutdown of the fuel cell system 100 g.Therefore, the same advantageous effects as accomplished by the presentembodiment can be accomplished.

In addition, although in the present embodiment, the description hasbeen made with respect to an embodiment in which the dew point sensor 21a and the dew point sensor 21 c are used to detect the fuel gas dewpoint Tda and the oxidizing gas dew point Tdc, such an embodiment shouldnot be deemed limitative. For example, it may be possible to employ anembodiment that uses, as the fuel gas dew point Tda, a dew pointcalculated based on parameters such as the amount of hydrogen suppliedfrom the hydrogen cylinder 30, the amount of water supplied from thewater tank 29, or other like parameter. In this case, based on the valuedetected by the temperature sensor 22 and on the elapsed time since thestart of the supply of water from the water tank 29, the controller 20makes a decision on the magnitude relation between the fuel gas dewpoint Tda and the temperature, Tcell, of the fuel cell 11. Besides, forexample, it may be possible to employ an embodiment that uses, as theoxidizing gas dew point Tdc, a dew point calculated based on parameterssuch as the amount of oxidizing gas supplied from the oxidizing gassupplier 17, the amount of water supplied from the water tank 29, orother like parameter. In this case, based on the value detected by thetemperature sensor 22 and on the elapsed time since the start of thesupply of water from the water tank 29, the controller 20 makes adecision on the magnitude relation between the oxidizing gas dew pointTdc and the temperature, Tcell, of the fuel cell 11.

Fourth Embodiment

In the third embodiment of the present invention, the description hasbeen made with respect to an embodiment in which the fuel cell system isequipped with a hydrogen cylinder. Contrary to this embodiment, in thecase where hydrogen supplied from a hydrogen cylinder is supplied withwater from a water tank, it is much preferable to employ an embodimentin which water from the water tank is first heated to appropriatetemperature and then supplied to hydrogen from the hydrogen cylinder inthe light of effectively increasing the dew point, Tda, of hydrogen(fuel gas) and the dew point, Tdc, of oxidizing gas. Therefore, in thefourth embodiment of the present invention, a description will be madewith respect to an embodiment in which the fuel cell system includes, inaddition to the hydrogen cylinder, a heating means for heating waterfrom the water tank, and with respect to an embodiment in which the fuelcell system includes, in addition to the fuel gas supplier, a heatingmeans for heating water from the water tank.

In the first place, referring to FIG. 16, a first configuration of thefuel cell system according to the fourth embodiment of the presentinvention and its operation will be described.

FIG. 16 is a block diagram schematically illustrating the firstconfiguration of the fuel cell system according to the fourth embodimentof the present invention. It should be noted that FIG. 16 selectivelyshows only constituent components necessary for describing the presentinvention, and the diagrammatic representation of the remaining otherconstituent components is omitted. Besides, also in the presentembodiment, the cross sectional structure of the fuel cell provided inthe fuel cell system is exactly the same as the cross sectionalstructure of the fuel cell provided in the fuel cell system according tothe first embodiment. Therefore, in the following description, adescription with regard to the cross sectional structure of the fuelcell provided in the fuel cell system is omitted.

Besides, the first configuration of the fuel cell system according tothe present embodiment is the same as the fuel cell system 100 f shownin FIG. 14, with the exception of the following differences: in additionto the configuration of the fuel cell system 100 f shown in FIG. 14,there is further provided a heat exchanger; water supplied from thewater tank 29 is heated in the heat exchanger; and the water thus heatedis supplied to hydrogen from the hydrogen cylinder 30. Accordingly,here, a description will be made with respect to the difference betweenthe first configuration of the fuel cell system and the configuration ofthe fuel cell system 100 f shown in FIG. 14, and a description withrespect to portions in common therebetween is omitted.

As shown in FIG. 16, in a fuel cell system 100 h according to thepresent embodiment, the hydrogen as the fuel gas is supplied from thehydrogen cylinder 30 as a fuel gas supply unit through the on-off valve41 as a hydrogen supply mechanism to the dew point sensor 21 a. Here,the operation of the on-off valve 41 is controlled by the controller 20,whereby the amount of or the rate of flow of hydrogen supplied to thedew point sensor 21 a from the hydrogen cylinder 30 is controlled.Besides, in the fuel cell system 100 f, the oxidizing gas is supplied tothe dew point sensor 21 c from the oxidizing gas supplier 17. Meanwhile,as shown in FIG. 16, in the fuel cell system 100 h the water from thewater tank 29 is supplied through the water supply pump 28 and thenthrough the heat exchanger 42 to hydrogen (from the hydrogen cylinder30) past the dew point sensor 21 a. Here, the heat exchanger 42 isconfigured such that the exchange of heat is effected between coolingmedium circulating between the temperature control device 19 and thefuel cell 11 and water discharged from the water supply pump 28 wherebythe water supplied from the water tank 29 is heated. That is, the fuelcell system 100 h according to the present embodiment is configured suchthat the water supplied from the water tank 29 and heated by the heatexchanger 42 can be supplied to hydrogen after passage through the dewpoint sensor 21 a. Here, the operation of the water supply pump 28 iscontrolled by the controller 20, whereby the amount of water suppliedfrom the water tank 29 to the heat exchanger 42 is controlled.

And, during the electric power generation of the fuel cell 11, theon-off valve 41 is opened to a predetermined opening degree, and thehydrogen as a fuel gas is supplied through the dew point sensor 21 a andthen through the fuel gas supply path 12 to the anode of the fuel cell11 from the hydrogen cylinder 30. At this time, there is no inflow ofhydrogen to the water tank 29 from the hydrogen cylinder 30 because thewater supply pump 28 is shut down. Besides, during the electric powergeneration of the fuel cell 11, the oxidizing gas is supplied throughthe dew point sensor 21 c to the cathode of the fuel cell 11 from theoxidizing gas supplier 17.

Besides, when supplying water to the hydrogen supplied from hydrogencylinder 30 during the shutdown operation of the fuel cell system 100 h,the controller 20 causes the water supply pump 28 to operate so thatwater is force-fed from the water tank 29 to downstream of the dew pointsensor 21 a. This results in the supply of water heated by the heatexchanger 42 to hydrogen from the hydrogen cylinder 30, whereby the dewpoint of fuel gas supplied to the fuel cell 11 is effectively increased.In addition, the amount of water supplied to hydrogen from the hydrogencylinder 30 is regulated by the force-feed capability of the watersupply pump 28, as in the case of the third embodiment. Besides, at thistime, although the on-off valve 41 is opened to a predetermined openingdegree, there is no inflow of water to the hydrogen cylinder 30 becausethe hydrogen is supplied to the dew point sensor 21 a from the hydrogencylinder 30.

Besides, when stopping the supply of water to hydrogen from the hydrogencylinder 30 during the shutdown operation of the fuel cell system 100 h,the controller 20 causes the water supply pump 28 to stop operating tothereby stop the supply of water to hydrogen from the hydrogen cylinder30. As described above, during the shutdown operation and the shutdownof the fuel cell system 100 h, the polymer electrolyte membrane 1 of thefuel cell 11 is well humidified by moisture contained in hydrogen as afuel gas and will be maintained in an adequate humidification state.Therefore, even by the present embodiment, the degradation of thepolymer electrolyte membrane 1 mounted in the fuel cell 11 is suppressedto thereby contribute to the extension of the life of the fuel cell 11.

Besides, although in the present embodiment the description has beenmade with respect to an embodiment in which hydrogen from the hydrogencylinder 30 is supplied with liquid, heated water, such an embodimentshould not be deemed limitative. For example, it may be possible toemploy an embodiment in which the fuel cell system 100 h includes, atthe downstream side of the heat exchanger 42, a spray for atomizingliquid water, and heated water atomized by the spray is fed to hydrogenfrom the hydrogen cylinder 30. Even in such an embodiment, the sameadvantageous effects as accomplished by the present embodiment can beaccomplished.

In addition, the position of the exchange of heat between cooling mediumcirculating between the temperature control device 19 and the fuel cell11 and water fed from the water tank 29 (i.e., the position where theheat exchanger 42 is disposed) in the fuel cell system 100 h accordingto the present embodiment may be any position as long as such heatexchange is effected. However, in the light of effectively heating waterfrom the water tank 29, the heat exchanger 42 is preferably disposed ata position of the cooling medium circulating path where the coolingmedium has a higher level of temperature. That is, it is preferred thatthe heat exchanger 42 be disposed at a portion of the cooling mediumcirculating path on the side of the outlet port of the fuel cell 11 andon the side of the inlet port of the temperature control device 19. Thismakes it possible for the cooling medium to more effectively heat waterfrom the water tank 29 in the fuel cell system 100 h.

Besides, the rest is the same as the first to the third embodiments.

Next, referring to FIG. 17, a description will be made with respect to asecond configuration of the fuel cell system according to the fourthembodiment and its operation.

FIG. 17 is a block diagram schematically illustrating a secondconfiguration of the fuel cell system according to the fourth embodimentof the present invention.

As can be seen from FIG. 17, a fuel cell system 100 i according to thefourth embodiment of the present invention has a configuration basicallyidentical with the configuration of the fuel cell system 100 f (see FIG.14). That is, the fuel cell system 100 i includes a fuel gas supplier 16(as a substitute for the hydrogen cylinder 30), an on-off valve 26 (as asubstitute for the on-off valve 41), a water tank 29, a water supplypump 28, an oxidizing gas supplier 17, a dew point sensor 21 a, a dewpoint sensor 21 c, a fuel cell 11, a temperature control device 19, atemperature sensor 22, and a controller 20. And, the fuel cell system100 i is configured such that the fuel gas is supplied, through the dewpoint sensor 21 a, to the fuel cell 11 from the fuel gas supplier 16while the oxidizing gas is supplied, through the dew point sensor 21 c,to the fuel cell 11 from the oxidizing gas supplier 17. Besides, thefuel cell system 100 i is configured such that the temperature of thefuel cell 11 is controlled by the temperature control device 19 and theoperations of the fuel cell system 100 i are suitably controlled by thecontroller 20.

On the other hand, contrary to the fuel cell system 100 h (see FIG. 16)that is configured such that, after being heated by the heat exchanger42 which uses the cooling medium of the fuel cell 11 as a heat source,water from the water tank 29 is fed to hydrogen from the hydrogencylinder 30, the fuel cell system 100 i according to the presentembodiment is configured such that, after being heated by the heatexchanger 42 which uses the reformer 23 as a heat source, water from thewater tank 29 is fed to hydrogen from the selective oxidation unit 25,as shown in FIG. 17. In addition, the rest is the same as in the fuelcell system 100 f shown in FIG. 16.

Even in such a configuration that the water is supplied, through thewater supply pump 28 and then through the heat exchange 42 whose heatsource is the reformer 23, to fuel gas from the fuel gas supplier 16,from the water tank 29, the polymer electrolyte membrane 1 of the fuelcell 11 is well humidified by moisture contained in the fuel gas duringthe shutdown operation and the shutdown of the fuel cell system 100 i.Therefore, even when employing such a configuration, it is possible toobtain the same advantageous effects as obtained by the fuel cell system100 h (see FIG. 16).

In addition, in the fuel cell system 100 h according to the presentembodiment, the description has been made with respect to an embodimentin which the dew point sensor 21 a and the dew point sensor 21 c areused to detect the fuel gas dew point Tda and the oxidizing gas dewpoint Tdc. However, such an embodiment should not be deemed limitative.For example, it is possible to employ an embodiment that uses, as thefuel gas dew point Tda, a dew point calculated based on parameters suchas the amount of hydrogen supplied from the hydrogen cylinder 30, theamount of water supplied from the water tank 29, and other likeparameter. In this case, based on the value detected by the temperaturesensor 22 and on the elapsed time since the start of the supply of waterfrom the water tank 29, the controller 20 makes a decision on themagnitude relation between the fuel gas dew point Tda and thetemperature, Tcell, of the fuel cell 11. Besides, for example, it ispossible to employ an embodiment that uses, as the oxidizing gas dewpoint Tdc, a dew point calculated based on parameters such as the amountof oxidizing gas supplied from the oxidizing gas supplier 17, the amountof water supplied from the water tank 29, and other like parameter. Inthis case, based on the value detected by the temperature sensor 22 andon the elapsed time since the start of the supply of water from thewater tank 29, the controller 20 makes a decision on the magnituderelation between the oxidizing gas dew point Tdc and the temperature,Tcell, of the fuel cell 11.

INDUSTRIAL APPLICABILITY

The fuel cell system according to the present invention and itsoperation method have industrial applicability as a fuel cell system ofhigh durability which is able to effectively suppress, by controllingthe dew point of fuel gas by a simple configuration, the degradation ofthe polymer electrolyte membrane when the polymer electrolyte fuel cell,which is operated in a low humidification state, changes its state tothe open circuit state, and as its operation method.

Besides, the fuel cell system according to the present invention and itsoperation method have industrial applicability as a fuel cell system andas its operation method for suitable use in an electric vehicle's driveelectric power source that requires high output characteristics andquick start-up as well as in a household cogeneration system thatrequires long term reliability.

1. A fuel cell system comprising: a fuel cell configured to generateelectric power using fuel gas containing hydrogen and oxidizing gascontaining oxygen; a fuel gas supplier configured to supply the fuel gasto said fuel cell; an oxidizing gas supplier configured to supply theoxidizing gas to said fuel cell; and a controller configured to at leastcontrol said fuel cell, said fuel gas supplier and said oxidizing gassupplier; wherein said fuel cell system further comprises a moisturesupply mechanism configured to supply moisture to at least one of ananode and a cathode of said fuel cell; and wherein said controller isconfigured to control said moisture supply the mechanism based on atleast one of either a dew point of the fuel gas or information relatedto the dew point of the fuel gas and either a dew point of the oxidizinggas or information related to the dew point of the oxidizing gas so asto increase at least one of the dew point of the fuel gas and the dewpoint of the oxidizing gas, in order to supply moisture to at least oneof said anode and said cathode of said fuel cell before cutting offelectric connection between said fuel cell and a load, and is configuredto thereafter cut off the electric connection between said fuel cell andsaid load.
 2. The fuel cell system as set forth in claim 1, wherein saidfuel gas supplier comprises: a reformer configured to generate a fuelgas containing carbon monoxide using a raw material through a reformingreaction, a shift converter configured to decrease carbon monoxide inthe fuel gas generated in said reformer through a shift reaction, and aselective oxidation unit configured to further decease the carbonmonoxide in the fuel gas with its carbon monoxide decreased in saidshift converter through a selective oxidation reaction, and wherein saidmoisture supply mechanism is a selective oxidation moisture supplymechanism configured to supply moisture to said selective oxidationunit.
 3. The fuel cell system as set forth in claim 2, wherein saidselective oxidation unit includes: a selective oxidation air supply pathused for supplying selective oxidation air to the fuel gas with itscarbon monoxide decreased in said shift converter; a mixing sectionconfigured to mix the selective oxidation air delivered through saidselective oxidation air supply path and the fuel gas with its carbonmonoxide decreased in said shift converter; and a selective oxidationcatalytic unit configured to, using a mixture gas of the fuel gas andthe selective oxidation air, obtained by mixing in said mixing section,reduce the carbon monoxide in the mixture gas through the selectiveoxidation reaction; wherein said selective oxidation moisture supplymechanism is configured to supply the moisture to either said selectiveoxidation air supply path or to said mixing section.
 4. The fuel cellsystem as set forth in claim 3, wherein said selective oxidationmoisture supply mechanism includes: a water tank configured to storewater; a moisture supply path for providing communication between saidwater tank and said selective oxidation unit; and a moisture contentregulation unit disposed in said moisture supply path.
 5. The fuel cellsystem as set forth in claim 2, further comprising: a selectiveoxidation air supply unit configured to supply selective oxidation airto said selective oxidation unit; wherein said selective oxidationmoisture supply mechanism includes: a cathode off-gas bypass path usedfor supplying cathode off-gas containing unused oxidizing gas dischargedfrom said fuel cell, to said selective oxidation unit; and a selectiveoxidation air regulation unit configured to supply at least one of thecathode off-gas delivered through said cathode off-gas bypass path andthe selective oxidation air supplied from said selective oxidation airsupply unit, to said selective oxidation unit.
 6. The fuel cell systemas set forth in claim 2, further comprising: a selective oxidation airsupply unit configured to supply selective oxidation air to saidselective oxidation unit, wherein said controller is configured tocontrol at least one of said selective-oxidization air supply unit andsaid selective oxidation moisture supply mechanism to make thetemperature of said selective oxidation unit equal to or higher than apredetermined threshold, when controlling said selective oxidationmoisture supply mechanism so that the moisture is supplied to theselective oxidation unit.
 7. The fuel cell system as set forth in claim2, wherein said controller is configured to perform control to causesaid selective oxidation unit to be filled therein with the fuel gas,after cutting off the electric connection between said fuel cell andsaid load.
 8. The fuel cell system as set forth in claim 1, furthercomprising: a water tank for storing water; a second moisture supplypath for providing communication between said water tank and at leastone of said anode and said cathode of said fuel cell; and a secondmoisture content regulation unit disposed in said second moisture supplypath; wherein said moisture supply mechanism is a fuel-cell moisturesupply mechanism configured to supply the moisture from the water tankto at least one of said anode and said cathode of said fuel cell.
 9. Thefuel cell system as set forth in claim 8, further comprising: atemperature control device configured to control a temperature of saidfuel cell, an annular heating medium path used for circulating a heatingmedium between said temperature control device and said fuel cell totransfer heat from said fuel cell to said temperature control device;and a heat exchanger; wherein said heat exchanger is configured toexchange heat between said annular heating medium path and said secondmoisture supply path.
 10. The fuel cell system as set forth in claim 8,wherein said fuel gas supplier comprises: a reformer configured togenerate a fuel gas containing carbon monoxide using a raw materialthrough a reforming reaction; and a heat exchanger; wherein said heatexchanger is configured to exchange heat between said reformer and saidsecond moisture supply path.
 11. The fuel cell system as set forth inclaim 1, wherein said controller is configured to perform control suchthat the dew point of the fuel gas and the dew point of the oxidizinggas are lower than the temperature of said fuel cell, during an electricpower generation operation of said fuel cell system.
 12. The fuel cellsystem as set forth in claim 1, wherein said controller is configured tocontrol said moisture supply mechanism based on at least one of eitherthe dew point of the fuel gas or the information related to the dewpoint of the fuel gas and either the dew point of the oxidizing gas orthe information related to the dew point of the oxidizing gas so as toincrease at least one of the dew point of the fuel gas and the dew pointof the oxidizing gas to cause the temperature of said fuel cell toconform to the dew point of at least one of the fuel gas and theoxidizing gas, in order to supply the moisture to at least one of saidanode and said cathode of said fuel cell before cutting off the electricconnection between said fuel cell and the load, and is configured tothereafter cut off the electric connection between said fuel cell andsaid load.
 13. The fuel cell system as set forth in claim 1, whereinsaid controller is configured to control said moisture supply mechanismbased on at least one of either the dew point of the fuel gas or theinformation related to the dew point of the fuel gas and either the dewpoint of the oxidizing gas or the information related to the dew pointof the oxidizing gas so as to increase at least one of the dew point ofthe fuel gas and the dew point of the oxidizing gas, in order to supplythe moisture to at least one of said anode and said cathode of said fuelcell before cutting off the electric connection between said fuel celland the load; and wherein said controller is configured to performcontrol such that the dew point of at least one of the fuel gas and theoxidizing gas is equal to or higher than the temperature of the fuelcell, when cutting off the electric connection between said fuel celland the load.
 14. A method of operating a fuel cell system including afuel cell configured to generate electric power using fuel gascontaining hydrogen and oxidizing gas containing oxygen; a fuel gassupplier configured to supply the fuel gas to said fuel cell; anoxidizing gas supplier configured to supply the oxidizing gas to saidfuel cell; and a controller configured to at least control said fuelcell, said fuel gas supplier and said oxidizing gas supplier; whereinsaid fuel cell system further comprises a moisture supply mechanismconfigured to supply moisture to at least one of an anode and a cathodeof said fuel cell; said method comprising: controlling said moisturesupply mechanism based on at least one of either a dew point of the fuelgas or information related to the dew point of the fuel gas and either adew point of the oxidizing gas or information related to the dew pointof the oxidizing gas so as to increase at least one of the dew point ofthe fuel gas and the dew point of the oxidizing gas, in order to supplythe moisture to at least one of said anode and said cathode of said fuelcell before cutting off electric connection between said fuel cell and aload, and thereafter cutting off the electric connection between saidfuel cell and said load.